I would guess you'd get 4 x 24 weekly injections. Decapeptyl also comes in 12 and 4 weekly doses, but I don't think the period is that critical that they'd worry about a few weeks. If you asked for another 12 or 4 weeks, they'd probably do it.
My oncologist wanted to know my PSA before agreeing to me coming off the injections, but that wouldn't have changed the duration by just some weeks, probably more like 6 months or a year, if I'd needed to stay on it longer. I did elect to do another 12 weeks on Zoladex after I was told I could stop, as it wasn't causing me any issues I couldn't put up with for 12 weeks longer, and it might give me another fraction of a % better chance of a cure.