This echoes my experience as well, however I prefer to break it into 3 x 80 minute blocks. The times when I've tried to push myself beyond that for a short per

This echoes my experience as well, however I prefer to break it into 3 x 80 minute blocks. The times when I've tried to push myself beyond that for a short period of time (a few weeks) I have ended up sick, ill or entirely unproductive to make up for it.