Most IT people in the Home Counties (the counties around London) commute into London, doing either car-train-tube or car-train-walk, so I would guess commutes average 60-90 minutes. Mine is currently 60 mins.

People within London will use normally use the tube (subway), but getting from affordable suburbs into the City also probably takes 60 mins. Personally, a 60 min journey which is mostly the train is bearable but mostly tube is not.

I've had a few contracts outside London with an average car journey of 30 minutes, which were good. However, anything longer is unlikely as contract rates drop off dramatically as soon as you go further away from London.