I think so long as you feel in control/comfortable with the terms, then go for it. I had a guy I'd seen twice ask me about a long term deal. Normally I would say no way Jose, on your way, but this is a guy I enjoy seeing, so I said 'ok I'll think about it and let you know'.
What I came up with was this:
1st return visit = normal price
Visits therafter a certain % off
The proviso was there must be no longer than a month between visits, or it reverts to normal price. So I shall see how it goes!
Funnily enough just had a 30 minute booking where the guy not only asked how many times could he come (my answer: I've never known a guy come more than once in 30 minutes) but also said he should've wanked before coming as he was so horny 