I agree on limiting time the amount of time you work per per day to maintain momentum, but for me (and perhaps some others), its also really helpful to limit the number of things you work on per day to cut down on context switching costs -- for three projects, spending 45 minutes per day working on one project (and rotating which one i work on each day) as opposed to 15 minutes per project each day. Thought this also strongly depends on the type of learning.