I wouldn't worry too much about cutting back as such, a lot of them will start to cut back and stop eating as much of them on their own. When this happens you'll find they won't really go for more than once a day.
However, I've seen people debate this between 1 year to 1.5 years. Going by breeding standards, you're generally looking at 1.5 years and I'd say that's a good measurement to go by and if your weirdie beardie stops eating so much before, then that's fair enough, if they're happy to be fed until then that's great