How Long to get through customs?

It will take at least 30 minutes to 2 hours.

I'd Book a 7 seater in advance
https://www.weknowlondon.com/category/car-transfer/?aff=heathrow

The arrivals hall at Heathrow is huge. I've gone through there about 6 times in the last two years...shortest was 15 minutes - I was incredibly lucky. The longest was about an hour. In previous trips before the last couple of years I've waited as long as about 90 minutes. It just depends on how many flights are arriving at the same time and how many of those passengers are also not citizens and have to go through the "All other passports" line.

And how do you know the flights going to arrive on time?

Last time (spring this year) I went through immigration in Heathrow, it took two hours.

The official stats for November say that about 90% of non EEA passport holders completed immigration formalities within 45 minutes. If that helps.

If you book a taxi (car service not a black cab) beforehand and provide them with your flight number they can track it online and will know if it's on time or not. They will also have a fair idea of how long it will take to get through immigration so will hold off entering the airport until the right time in order to save on parking costs.

A couple of the car services are www.justairports.com and www.simplyairports.co.uk.
I have seen both recommended here.

There are four terminals at Heathrow. Each has international arrivals. No one can predict how long it will take to get through immigration but minutes to two hours is about right.

As stated, when you reserve your ride from Heathrow make sure you are specific on the number of people and the number of bags. You don't want them showing up with a vehicle too small. And they must know the flight number so they know which terminal and if the flight is delayed.

Thank you for all the advice and recommendations for taxi services. I will be researching those options!

By "Friday at the end of March" do you mean March 29?

I arrived at 7 am on a Thursday in late August and was through passport control in about 20 minutes.

No, we’ll be arriving Friday March 22nd.

Like Anita, I got lucky a couple weeks ago and it only took 15 minutes to get through passport control at terminal 5. My longest wait to get through was close to 1.5 hours.

I was extremely lucky this past May and got through passport control in 5 minutes. Walked off the plane and directly up to an open kiosk. We had carry on luggage only so we were on our way.

How old are your children? If they are very young, you may be escorted through very quickly.

I just was there, arriving at Heathrow about 4 weeks ago.
My plane landed at 2:30 pm on a Thursday, and I finally got out of the airport at about 4:45 pm.
Then it took me about an hour and a half to get to my hotel on the Piccadilly Line Tube, to near Kings Cross.
The line for Immigration is just horrendous at Heathrow.
Make sure everyone visits the bathroom before you line up, as there's no way back!!!
And you will have to stand for about 2 hours.

I'll just add one thing that tripped up my brother and SIL and increased their time in the Immigration area. We arrived in May on a Delta flight. The Delta Flight Attendants who were passing out the landing cards during the flight said 1 per family not 1 each. That was wrong. The Immigration official sent brother/SIL (and many others I recognized from the flight) back to the writing desk to fill out another card for my SIL on her own, then they were back in line again. The official was very nice but said each person needed their own landing card and in fact my brother had questioned it when he filled out the card because there was not a place to add additional people.

Just mentioning this so things go smoothly at the Immigration desks. TBH, I've found the UK officials much nicer than the experience returning to the US with a US passport.

I had a speedy experience on a Tuesday in early November. Landed at 8:55am at Terminal 5 and walked right up to an immigration officer, no line (and I was near the back of the plane). My bag was already on the carousel waiting for me. Took Heathrow Express and bought my Oyster card at Paddington at 9:51am (thanks, receipt!). It definitely depends on the volume of flights that land around the time as your plane. It took well over an hour to get through immigration the last time I was there because 3 planes landed in short order of each other.

Our recent entry into Heathrow was seamless, and we were through passport control in about 20 minutes. Previous poster was correct, in that both of us completed a landing card and that made it faster. I had booked a vehicle from Blackberry Cars and they were prompt and professional. https://www.blackberrycars.com/ Do not use one of the black taxis, unless you want to burn through your money unnecessarily.

Do children need their own landing card as well? I’ll be traveling to London next summer with my 6 and 10 year olds. I’ve been before but never with children.

"It took well over an hour to get through immigration the last time I was there because 3 planes landed in short order of each other."

It's probably not just the number of aeroplanes (or how big they are), but where they came from. A jumbo jet from Ottawa packed with friendly Canadians is likely quicker to process than passengers on smaller craft from one of the more dodgy countries.

avab80: Every person entering needs a landing card. Adults and children.

Seconding SJ on finding a bathroom before you get to the passport control queue.

Biggest risk for long wait times is when multiple aircraft deboard passengers within a few minutes of each other at the same terminal. Most likely time for that to happen with flights out of North America is early morning. But Heathrow is the busiest international airport, flights get delayed, etc.

one of the more dodgy countries.

what's a dodgy country?

My parents, myself, and my three children will be traveling to London
at the end of March. Our flight will be arriving at 8:40am at Heathrow
on a Friday morning. Can anyone give me a rough idea of what to expect
for length of time to get through customs? I've only flown in to
Heathrow at 11pm and we moved fairly quickly through the line.

Will any of the 6 of you be declaring any items to be taxed? If not, then customs is a walk through. No need to worry about customs.

As for passport control, it could take 20 minutes; it could take over 2 hours. Both have happened to me with equal regularity. Whichever car service you choose, find out if they charge you after a fixed amount of wait time. Your flight could arrive late; passport control could take 2 hours.

So there you have it. You have been informed. It could be 15 minutes or it could be 2 hours.

A Custom gate site now is dependent on the size of the Airliner. And Airliners are now huge. Its a pig through the python problem. What we do is have a good long lunch/Dinner/Breakfast at the terminal and let everyone else stand in line. We don't stand in lines. We show up an hour later to customs, and there is usually no one there. We could have stood in line, or had a nice, though somewhat expensive meal. The time is about the same. Yet our strategy is more enjoyable.

Thank you all so much for your input. I truly appreciate it! Thanks especially for the tip about going to the bathroom before waiting in line! A must with the kids! They will be 11, 9, and 6 when we travel. I also will look into the transportation companies that were recommended. We'll set one up in advance that will be big enough for our group and our bags. Thanks again!

A must with the kids! They will be 11, 9, and 6 when we travel.

With kids that age you'll likely find yourself hauled out of the normal queue and into a fast track queue dedicated for families with young children and others who need assistance. I still would take advantage of the toilet break.