I've never kept score to be honest.
Generally, if someone lasts a month, they last a year. I have quite a few that have been there well over 2 years, Jimmy, Sean, Nick, Kat, Mick, Jason, Jordy, Vicki and a heap that have been there over a year, too many to list. Over 75% of my current clients.
I also have some that have stopped then come back, Brian and Josh just started back after an 8 and 12 month hiatus.
You'll find out how fickle some can be soon enough