Have them offer a referral on TER, which is a permanent sort of referral other providers can see.

but the passage of time in between asking and having seen the provider from whom you are requesting the reference.

Somewhere around 2 to 3 months seems to be the cutoff in most cases.

If you have a regular whom you see on a fairly frequent basis, I would think she'd have to problem refering you to any number of providers, though I would hope that some providers will chime in if this is not the case.

I know that I have done this with my regulars and never had any negative comments regarding this.

constantly seeing new women, you should constantly have new references to give. I don't mind giving references but 3-4 is the max times I want to give a reference for someone I've met once. If you're a regular, that's different.