I don't set any time limit, i just go with the flow. I didn't reflect about it before, but I think it's a good idea. Now, I will set an alarm clock, and add it to my timetable
I like to use the 25 min and 45 min deep work timers. They give me that sense of ritual and structure. If I am in the mood for more, I reset the timer and do it for longer. Having the timer end just makes me aware of my accomplishment and deepens the habit. I am a writer, and so I am no stranger to deep work, but I have noticed since doing this it takes part of "the load" off my shoulders that *I am* actually working and have proof. Writers have that doubt. But now I don't have that responsibility. The timer does! My work has improved a lot because of it!