I am no longer in chastity now, but in my last relationship we did not have a limit of orgasms in a certain time etc. At first she would let me out to fuck her or get a BJ once a week or so, but as time went by and she got more and more active with other men, time between my orgasms grew bigger and bigger. We played more with ruined orgasms and so on. So the last year of our relationship it was more like a ruined orgasm once or twice a month.
After we broke up, we decided that she should still hold my key and that my chastity should be permanent for as long as both wanted. I played a lot with dildos from then on, and quickly learned how to orgasm only from anal stimulation. I had no restrictions on how often I was allowed to do this.
But my experience is that after an sissygasm, you are just as horny as you was before. So my point is, if you teach him how to sissygasm, you can drain him quite often and still get the benefits from chastity