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While we are at it, I think one of the important aspects of learning is periodic recall to ensure you do not forget what you learn.
Especially this mode of learning tiny bits when you have time implies:
1. You are not part of a rigorous system like school/college that ensures learning has happened.
2. Your learning is not periodically assessed through tests and applied through assignments.
3. It takes lots of effort and discipline to ensure what you learn next builds up on what you learnt previously and not a bunch of random stuff that do not amount to anything much at the end of a year.
So definitely this is a very arduous undertaking.
With all these in mind, Udacity and Coursera offers mini-degrees which are a set of 4 to 5 related courses on a subject. Coursera atleast allows you audit those courses for free.
If anyone has a better suggestion, I would be very interested to know.