It all depends on the type of community, and the expectations of its members (how's that for a consultant's answer? ). The most common one I see is 24 business hours when the expectation is that SMEs are the ones who answer questions. When the expectation is that "someone from the community" will answer when they get around to it, I don't normally see any kind of stated SLA (which is what you experienced here).
For what you are doing, I'd ask your SMEs if 24 hours is enough time for them. Do they frequently travel? If so, perhaps 48 hours might be better suited. Whatever your SMEs feel they can comfortably meet, just make sure the SLA is communicated and easily found in your community site.
Please, let us know what your community decides to do!