Yeh its pretty intensive, I think at the moment Ive completed less than 40% of the 48 hour gamejams I've participated in. I only leave my PC to sleep or get some exercise, all other plans get cancelled, etc, sometimes I take the following day off work for recovery (since the final night is best to pull and all-nighter). It can be a very rewarding experience (also super frustrating when things dont go well) but definetly give it a go sometime if youre feeling up for one!