It depends on both the agency and the agent.
At most agencies, I suspect there are agents that respond to e-mails and/or phone calls very quickly and those that check their e-mail less frequently. It would be hard for the agency to mandate a policy on this because some days are going to be more busy than others, etc.
The other issue is how long it took the agency to assign the matter to an agent, and then that agent's response time.
What I'm used to seeing--if I send something to the general agency address, I get a "same day" or "next morning" response. If I send something to a specific agent, within a few hours I either have a response or I get an "out of office" message right away. In that situation, I can e-mail another agent or call the agency number.
Some agencies handle everything thru your specifically assigned agent, others have an agent "on call" to handle new things
Guess what I'm saying is that it is an internal matter based on how the agency does business. BUT, when all is said and done, I'd expect a response within 24 hours on a weekday. Even if the response is "thank you for your inquiry. We are working on it and will be back with you later today," that's a response and a commitment.