Every agent is different, and works differently. Some who use smartphones and pda's may receive your message and return the call within 15 or 30 minutes. There are other agents who are working part time, or who have limited access to emails who take days to return a call.
Don't work with an agent who doesn't return calls promptly. If you called or emailed someone and they don't respond in a timely manner, find another agent who does.
I was on a listing appointment yesterday and the seller called 2 of us. I responded in 30 minutes. When I met with the seller, it was a week later and that other agent still did not return her original call. Not good.