I usually schedule 30 minutes between clients so I have time to chat with each client and deal with anything that might "come up" during their session. I am solo and so I also need the time to return phone calls, schedule appts, change the linens, etc.
I just wanted to add that I have had clients point out that they never feel rushed when they come to me, and they feel like I am all about them while they are there (because it's rare that the next client shows up while first client is still there). As stated in an earlier post, if I scheduled them closer together I could get more people in, but I like my clients to feel like they are my #1 priority.