3h for me ATM. Has been like this for a few months and is surprisingly effective.

I do personal projects on the train mostly. The only nuisances are obscure programming references and pip. Can generally move to another problem if I'm blocked though.

I'm lucky, work for smart people who realized that if they wanted me to sign they had to yield on where. (Or as they put it: we don't understand what you do anyway and we trust you, so whatever you want as long as you show up most of the days.)

Edit: Used to work on personal projects in a previous job that included long commutes. For me it is often the best time of the day for programming: Nobody to interrupt, few distractions, a very specific start time and stop time.