On my VFD, members are assigned to one of three crews which rotate the weekend duty. Essentially, every third weekend, you have to be in town and respond to all calls from 6 PM Friday to 6 AM Monday.
For weeknights, we have a monthly sign-up that's posted at least one or two months in advance for the positions of Ambulance, EMT, Utility and Engineer. You're have to sign-up for a few weeknight shifts for whatever you're qualified for...if you don't, you're assigned to whatever slots are unfilled. I generally sign-up for two ambulance shifts and one EMT shift per month, though I usually end up covering a couple other full or partial shifts in the course of a month.
I should point out that we don't have any sort of "live-in" arrangement. If you have a night or weekend shift with the ambulance or utility, you take it home with you and respond from home (most everybody on the department lives in town, and only those who do take the ambulance and utility).
As far as daytime goes, we have two paid guys in the Fire Prevention Bureau who cover the ambulance during the day. There's a handful of us (myself included) who work in town all or part of the time (I'm usually in town two or three days minimum most weeks) and are able to respond.
There are minimum requirements for call attendance: 25% for probies, 15% for regular members and 8% for reserve members. This ends up working out to about 10, 6 and 3 calls per month, respectively, which is not hard to make so long as you don't ONLY respond when you're "on duty".