We've had two sessions a week for about 6 years. This is how I've found it works for us (similiar to Daniel):
Two-hour pose with a timed 5 minute break every twenty minutes. $15+ for portrait (they just show up); $25+ for costume (they come up with interesting outfit); and $35+ for nude. The plus is tips. The bigger the class the better they do. So a figure model can make over $45 in two hours. Once a year I take out a small ad in a local paper for models.
I've found it's important to start and end on time so noone is inconvenienced. Also, if I have to cancel a session for any reason attendance will start to lag. Our group is popular and usually packs a room because we are informal and there's lots of yacking. This has taught me to paint and talk at the same time
as well. We rent space for $10 a session at an old train depot, at night with artificial light, and an old firehouse, in the morning with daylight.
As solitary artists, it's a good chance to get together, have a good time and still get something accomplished.