I was once part of a group that volunteered at a shelter for an afternoon (as part of a community-service project). But that wasn't very useful to the shelter; they have mandatory training programs for volunteers and none of us had gone through that, so we ended up spending the afternoon at a bunny romp. That was fun, but I didn't feel like I was helping them -- more like staying out of their way. (I'd been expecting to spend the afternoon cleaning cages or walking dogs or doing data-entry.)

If I wanted to volunteer usefully at a shelter, how is that time best structured? I work full-time and have other obligations; would a few hours once or twice a month be at all useful, or is that below the "thanks for the thought, but..." line? What are the typical time commitments that shelters look for from volunteers?

(I realize that I can ask my local shelter about their needs, but that could be an awkward conversation if it turns out they need more than I can offer. So I'm asking here to get a sense of what's typical.)