I would always contact a lady before using her as a reference. First of all it's the polite thing to do and secondly it will help jog her memory as to who you are...

It depends on how often you met & what the most recent was but I'd think 6 months to a year.
skb

use me in the bedroom, then use me as a reference, and I am perfectly happy with all of it! Once I give you the word, you dont need to later ask for permission.

and let the lady know who would be contacting her.  Otherwise, how will the provider know it's not the guy's GF, SO, PI?  Let's be smart here.
skb