I did Cambridge/Cambridge North into Kings Cross for just over 3 years. The train time itself isn’t all of it though, as you’ve got overheads each end. In Cambridge I cycled to the station, so had to allow 10-25 mins (depending on station) to get to the station, then time to lock the bike, time to get to the platform for the train etc. Once you get to London it depends how far from Kings Cross you are, walking distance is ok but the tubes can be awful depending on the line. So train is about an hour, plus 30 mins Cambridge end and 10 mins London end and you’re already looking at 2 hours there and maybe a bit less home but an increased chance of having to stand for most of the journey. It was nearer to a 12 hour day than a 10 hour day most days, not allowing for the bad journey you get about once or twice a month or if you want to go out with colleagues after work or work late.
In the end I decided less money was worth the hit, as quality of life was better without the commute. Now kids are in the mix I definitely wouldn’t do it again.