I often get local regulars phone first thing asking if they can come over "now", I don't count the shower as that's already done so its usually a couple of minutes for make up (light foundation, non smudge lippy and mascara) and on with the stockings and heels etc.
I wouldn't do 15 minutes in between bookings as I'm a lazy whore who likes a good break in between clients so I'd say ideal for me is one first thing in the morning and another around lunchtime as this allows me a time after the first one to re shower, remove the makeup and do other things so if I do get a second client I'm already showered so again I just need to freshen up, mouthwash, quickly re apply a bit of makeup and throw on my stockings again.