Time required for AoPS depends a lot on how many of the problems you need/ choose to do. If you already know a lot of the material and just want to add depth, you can jump ahead to the course homework and challenge problems; 3-6 hours per chapter (Algebra was usually about a chapter per week) might do it. However, if you are learning new concepts and need to work through the regular problems in each section, as well as those at the end of the chapter, and then do the chapter's challenge problems, and also do the assigned Alcumus problems, plus that week's homework assignment... it could take much longer. The whole point of AoPS is to throw problems at you that you need to spend a lot of time thinking about, so it does that job best when you allow yourself enough time to think, and to try all the problems.
ETA: which is why we switched from on-line course to self-paced home study with Mom - we just needed more flexibility in the after-schooling schedule
