it would be impractical. why would you want to stay in london though anyway? surely the cambridge colleges etc are an experience that you'd want to have if you went there?
About an hour to and hour and a half by train, anything from about 2 hours to 4 hours
why are people just making up times?
the fastest train (from kings cross) is 47 mins. the majority of trains are fast. there are slower trains too, from liverpool st particularly.
As for living in London and commuting, I think if you can get away with the whole keeping term thing, it's not impossible. You'd have to live within walking distance of king's cross. You'd also need to do an arts subject that didn't have 9ams everyday. Lots of supervisors/fellows do it. The main problem is that cambridge train station is not close enough to faculties. I think I would have tried to do it this year, except the whole point of being in london would be to be able to enjoy the nightlife, which couldn't happen with a cambridge workload.