Before I moved this year I was seeing 90% regulars as I just daren't risk no-shows with being really busy, but before I'd say I was about the same as Amy and one-third of bookies would be newbies. As said, some men never return to an escort, and I would get a lot of people passing near me on the M25. Plus, as a mature BBW I realise I'll be a one-off 'experience' for some clients.
I think you should be worried if you NEVER get returnees, but on the other hand I'd say up to 50% newbies is fine too.