You've been volunteered

Volunteering is one of the best methods for skill-building, especially when you lack essential library skills...answering questions, teaching, perhaps some technical skills that you had to skim in school. But you can sometimes hit a point where your volunteer hours have taken you over and you need to *gasp* quit.

1. You can't get your homework done. Some readers may have made the decision to let their marks slide a little to get some experience...or some shut-eye. If your marks really start to suffer and you pass an as-low-as-you-go point, ask for some time off.
2. You're working extra shifts. Repeatedly. It happens once, an extra hour in their busy time, or picking up a shift for another volunteer, but then it becomes your regular gig and you can feel the cold, guilty chill when they ask you where you were on Tuesday when you are only scheduled on Thursday. A few extra hours, ok, and just mannerly of ya, but sit them down with a calendar if this begins to happen every week. Especially during Midterm Week (see point 1).
3. You're doing paid work for free. If you start answering phones or covering for the Chairman's administrator, outside of your regular, library-like chores, you're doing paid work for free. Occasionally: good sport! Every damn day: check the agreement you originally had with the organization and remind them that you have other duties they need done in the limited time you are there. Unfortunately, point three is pretty sticky, since you may have other duties as required written into your volunteer contract and they might call these duties other. I'm betting though you are pretty uncomfortable with some of these extra duties since you may not spend enough time at the organization to do them really well and this may, unfairly, affect your final evaluation. Get this cleared up as soon as you can, nicely.
4. Volunteering is interfering with your paid gig. This is a tough one if your paid job is slinging coffee and your volunteer work is in library land. You have to pay your bills, but if it means that much to you, can you work out a paid position in library land? (See point 3 if you suspect they can afford it)

As a serial volunteer, I have hit the over-scheduled point many times. I'm still not good at saying when; after all, I volunteered. But if you see those signs, you need to renegotiate your commitment or quit. Consider this another experience from volunteering: recognizing an overly demanding job that may not get you where you want in your career.