A good WG will give you back the time you/they spent showing before and after (or in a Soho walk-up, the time they spent with the maid while you undress). However it won't matter so much once the girl walks in as with experience, you'll be able to tell the outcome of the punt after the first few minutes. Either (a) she comes in asking for money, having bad/apathetic attitude, and not engaging you, or (b) she is really lovely, cuddly, friendly, and putting herself out to give you a good service.
If she's asking for money without giving a hug or saying a word to you, you can bet she'll be calling time and telling you to hurry up 5 minutes to the hour.