We do one unit a week and take every 4th week off so it lasts us 38 weeks, which is about the length of a typical PS year. I think it is designed to be one unit per grade. Primer being K, Alpha first, etc.
Except I have been told as you get into higher books each book has material that would be considered in a ps to be at varying grade levels. So while in ps a kid may be in 4th grade, in MUS he may be doing work considered to be 2-6th grade. So you can't think of those book levels as grades. It may take a year but then again it may take only 6 months or even 2 years to complete.
DS2 did Primer in about 4 months also. Alpha we are right on target with one unit a week. DS1 Has done Alpha and Beta and is doing one unit a week also. Sometimes he finishes the unit early but we still don't start the next unit until Monday!
I actually have paid no attention to how long it takes so I'm not real sure. My dd has gone through Primer and is into Alpha and that's been within the last several months. My ds did Alpha and is now into Beta and looks like he will be fininshing Beta by the end of this school year but I can't remember for sure when he started it. I think late fall -- maybe around November?? We just work at each lesson until ds gets it no matter how long that takes or how quickly it might take. I guess it's working out for us though to take a year or a little less with each book.
I don't schedule MUS. Since Alpha and above are mastery, we stay on a leson until she is done and move on. Additionally, I let her work completely at her own pace. She usually does a page or two, Friday she did 8. A level can last two months or it can last two years.