I can only speak from my perspective as a client.  I try to respond as quickly as possible, especially when making initial contact and setting up an appointment.  I would say I respond within 10-15 minutes, unless I am working where I need to be more discreet.  Even then, it would likely be within 1-1.5 hours.  The anticipation kills me and focuses my attention to get it on the books so I can move on to other things.

With providers, everyone is different.  Some leave you hanging for days before getting back.  Some don't even respond.  Some are very prompt, and that is always noted and very much appreciated.  Communicating quickly is a sign of respect from both parties.  I will say it does drive me a little crazy inside when it takes a long time to hear back.  But I also understand that I have no idea what's going on with them (another client, day off, me time, gym, sleep, etc.).  I can only imagine they are getting bombarded all day, every day, which can't be fun.  It's just me being anxious and excited to spend time with someone.  I've never let it show or mentioned it to them.  

I do my best to be quick on my end, and that's really all I can control.