Not technically modding but I clocked up a marathon 19-hour stretch (on more than one occasion) including coffee and lunch at the desk on Generals with EA in the summer of 2001
If you speak to EJAH, he's literally non-stop - hes permanently working, EA even shipped all the PC etc to his house. No joke.
In modding terms, about an hour or two at the end of each day - Ive learned over time that sometimes once you step back from a problem for a short while, a solution usually presents itself. Previously, I would hammer away non-stop until I fixed something but I found that would mainly introduce new problems in itself - the breaks are essential, not just for your sanity and wellbeing but for your productivity too. AT w/ends, Im usually playing/modding all the time.
Im lucky - I spend about 40% time coding as I usually know what I want to do and how to do it, 60% is testing/troubleshooting.
[note to self: get out more].