how long to get from airplane to exit in LHR?

There are lots of variables, the main one being which gate you arrive at. A long walk can slow you down a lot. Another is whether you're in business or economy.

Based on my general experience, I'd allow 45 minutes from disembarking the aircraft to exiting through the Customs green channel, and another 15 minutes ro the bus station, assuming you have checked luggage.

Other possible delays - they can be slow in unloading checked luggage, and at a busy time like you're ariving at (lots of Asian flights arriving) there can be a backlog for the lifts as you head to the bus station.

Probably I would plan at least an hour but not really an answerable question - so many variables especially for passport control (even with the quicker automated entry system for US passports, it can take a while). Heathrow is just a massive airport - while I'm not sure where bus connection is, the train station is quite a trek after passport control.

For Heathrow to Victoria you totally don't need to book ahead. Just buy for the first available bus when you arrive. There is always lots of capacity. All buses have arrived from elsewhere and empty out on arrival.

Other variables are your flight departing on time, landing on time without having to circle the airport, and whether you pull right up to a jetway where you walk off the plane directly or whether you park at a hard stand and need to take a bus to the terminal.

"while I'm not sure where bus connection is, the train station is quite a trek after passport control.'

It's just above the tube station (terminals 2/3), so roughly the same in terms of time, assuming that's where the OP is arriving.

And I agree there's no point in prebooking. It's not like you're not going to get a seat.

Generally I find Heathrow quite good, but sometimes things come up. On a recent trip the luggage belt broke down and we waited an hour while they fixed it.

Which terminal at Heathrow?

There are so many variables (not least your flight leaving on time) that it is very hard to say. Heathrow is an extremely busy airport, and for flights that arrive in the morning more likely than not you will get stacked up into a holding pattern circling London till there is a landing slot available. This obviously adds additional time (and feels frustrating when you are looking forward to getting off a long flight and see yourself going in circles getting closer and then further away from the airport on the in-flight map). However, if it's a clear morning and you're sat near a window you do get some great views of London.

The time of day and how many other flights arrive at the same time and from where all affect the time through immigration and customs. Just get you ticket when you get to the bus station or get a ticket that is good all day such as the Heathrow Express. Any transportation running from the airport knows this

Thank you for all the comments! First time flying to Heathrow and trying to get an idea of what to expect. Glad to hear the National Express buses aren't usually full. I'll take the wisdom from those who have "been there, done that": won't make a reservation but will just in line for the next available bus - with the hopes that I won't be in the air circling London for hours.

“Hopefully not circling for hours…”
Been there, done that or at least it felt like it ……but if you’re on the side of the plane that looks down on London as you circle, you get some great views!

I just took a National Express bus from Heathrow to Liverpool the Sunday before Christmas this past December. I padded our time to get through the airport assuming it would be one of the busiest travel days of the year and hilariously, it was the fastest we've ever gotten through an airport in our lives. One word of advice- find a Youtube video that walks you through how to get to the National Express waiting area from the terminal in which you'll be arriving. I did that in advance and it still felt like a much longer walk than you'd expect, I think it took almost 20 minutes. You have to take a few elevators and you feel like you're on a journey to Mordor. Also, the National Express waiting area is small and doesn't have a lot of options for food/drink, there's a good coffee shop and a general convenience store, but that's about it, so if you think you're going to need snacks try to get some in the airport terminal before you head to the bus area. Even on a weekend before a big holiday the bus we took was busy but not completely full.