Much like the above poster I also alternate between new girls and girls I've seen previously
I punt about twice a month, and have two separate hotlists, one for girls I've seen, one for girl's I've not.
Even my favourites I'll only see once / maybe twice a year. I don't go back so often I have to worry about them being put off.
I guess it would vary from girl to girl, but unless you're seeing a girl more than once a week, they'll probably be happy for the repeat business.