When you try to do as many things as I am trying to do, you quickly find out how long you must spend on individual activities when you choose to perform them in order to attain peak productivity. It probably depends on several things, including individual skill level and approach, but here are the times I’ve found best for myself:

Computer Science Research – 3-5 hours.
Mathematical Research – 4+ hours (as long as the problem continues to interest you and you’re not running against dead ends), with breaks approx. every 2 hours.
Programming – 4-6 hours, or until the problem is solved.
Web Design – 1-4 hours.
Social Sciences and Technical Writing – 2-3 hours.
Philosophy and Creative Writing – as long as the ideas keep coming.
Musical Composition – all day, with breaks but no interruption for other skills.
Piano practice – 1-3 hours.
Generating ideas – none at all; this should be a passive activity.