Usually, you should have a response the same day or within the next couple of days. Unfortunately, some agents don't think it is important for them to give feedback. No amount of phone calls and emails will change that.
Sometimes, if a client would possibly be in from out of town and looking at several properties over a short time, the response time might be longer due to the agent focusing on selling your house to whom he is showing it.
How long has it been since it was shown and what is your agent telling you?