Really depends on your starting skill level and the actual content of the course. I went through Algebra 1 when I was taking Algebra 2 at school, and got through it in a month or so without setting much time aside for it. On the other hand, I've been working on AP Calculus BC on my own time for about 2 months and am barely through with the first unit.

It also depends on how you pace yourself! I mean, most learners go slower than others and left behind while the other are ahead! It determines how you observe yourself while solving practice problems. If you take your time timing yourself later, you'll find out how long it takes you to get through a course. So, therefore, there isn't an estimated time!