Them ones I cut it yesterday because I just felt like It was too long and patchy and it gave me a bad itch as Well nglExactly how long kinda depends on genetics. First and foremost, are you the only one in your family that lacks a beard?
If so the problem is external, if not, genetics. Diet is also important for more hair growth, so have a balanced diet, don't know the exact amount of protein and iron you need, you'll have to figure that out yourself.
But in short, eat good and cut it when you feel like it won't grow anymore, that's when you cut it. It's a feeling, trust your gut.