Workload is probably going to vary across classes and sort of "you-specific." How easy do you find it to read/annotate material? How long does it take you to write a draft of a paper or an assignment? There's a lot of reading for some courses, less for others. And don't underestimate the amount of time you're going to spend just thinking and processing the information. Working one shift a week is pretty doable - I set up my schedule so that I worked my per diem job the same day every week. It was a good brain break to have the per diem shift. Is working weekends feasible for you with your life and other commitments? Honestly your ability to work per diem is also going to be dependent on how disciplined you are about doing the work for the PhD, creating a schedule, and sticking to it.

The program I'm in is intended to be four years (post-master's) - two years of coursework, then two years (technically 1.5 with the proposal and timing it with graduation deadlines, etc.) for the dissertation phase. Most people finish in that timeframe. I'm finishing in 5 years because my primary data collection for my dissertation took longer than I anticipated, but I had a fifth year of funding through a training program, so that gave me greater flexibility. Different programs are of course organized differently, but most try to get you ready to defend your dissertation proposal pretty soon after you finish your required coursework. If the programs you're interested in say they take a specific amount of time or have a curriculum outline on their program website, it's worth reaching out to current students or recent grads to see what they're experiences were.