First thing is that all emails with a valid return address get a reply.
If you mean simple website requests like "Hey! This link doesn't work you jerk!" or "We've added your link to our directory, please add our link to yours!". It can wait up to a week. It really depends on what they are asking for.
I can leave these for up to 48 hours before I reply and still get a "thanks for the quick response" from the sender.
The delay on these for me is that they usually take a bit of thought.
If the email comes in and we're awake (we're in EU selling mostly to US) it is almost immediate. Our customers are quite often under a heavy deadline and are usually freaking out. We make it a point to calm them down as quickly as possible.
Our email support during the heavy sales time of the day is almost like IM.