I was going through a programming course from MIT, and they used some calculus stuff I didn't understand, either never learned or forgot in the last 35 years. So I looked up their calculus class and started on that. Nope, still too advanced, even though I got A's on it in high school. So now I'm doing a pre-calc class and things are coming back to me. Figure in 100 hours I ought to be back up to speed on calculus enough to finish the programming course.
I don't think I had a solid idea of a timeline, but I figured with a couple hours a day I could get through Fundamentals I-III in a month or two. How naive I was!