Most of our docs work 126 hours/mo for full time. I work a little over 150h/mo or 1800h/year though I don't think I'll be able to keep that up indefinitely. For the moment though, it has afforded me the ability to pay off 400K in education loans and is allowing me to play catch up in retirement savings. I try to invest about 8-10K/mo with my current level of compensation. We'll see how long I can keep that up. I could probably save more if I didn't take so many vacations, etc..
To get back to your questions though... The short of it is yes, you can work "part time" but you probably need to negotiate that up front with your employer during contract negotiations. It's difficult to suddenly go from 8/mo to say...14/mo or vice versa because your group is going to want to staff the ED appropriately depending on how many shifts need to be covered. However, I've found there is always some flexibility and our veterans have had no problems reducing their shift load because there are enough of us interested in picking up extra shifts. For instance, I probably work the most at 17/mo on average, but one of our veterans works about 10/mo and is planning on going down to 8/mo starting next year which hasn't caused any problems at all. He used to work 13-14/mo a few years ago and we still consider him FT.