How long does it take to get into London from Heathrow?

It can probably be done, but I wouldn't plan on it. Heathrow is a notoriously slow transfer airport.

I agree with the other poster about Heathrow being slow for connections - it's a huge airport. Walking from terminal 4 to terminal 1 takes about 45 minutes. Also, it takes at least 30 minutes (more if there is traffic) by cab to get to London and it cost about 60GBP.

You can take the Heathrow Express to London and back, less than 30 minutes each way, but it is expensive. Check the price on-line. You could also simply take the tube, which is cheaper, but takes about an hour each way.

Do you want to do this just for the experience of "being in London", or because you don't know what else to do at Heathrow between flights?

Remember, if you need to make an ongoing connection at Heathrow, you will be starting from scratch, and it can take up to 3 hours!

Especially if you have to go through Customs because I've been there where it's taken 15 minutes and I've been there where it's taken 90 minutes. If you happen to have the time, go ahead but don't risk it if it takes a long time to go through customs.

If you clear immigaration and customs quickly, you can take the Heathrow Express train (15 min.) to Paddington Station. There you can catch a Circle Line Tube to some of the major sites. Just leave LOTS of time for return!!!! I'd suggest you consider Windsor instead of London proper. I think you can catch a bus - or do a taxi. Windsor is one of my favorite places in the UK and 2-3 hours is enough to get a good taste of it.