Millet is supposed to be more of a treat than a daily staple. But if you don't give too much at a time I think it's ok to give more than once a month.
I know it's probably not the best thing, but I actually give each of mine about a 1/2 inch piece(depending on how fat the millet spray is) every night as a night night treat. They've come to expect it and even tho I don't give them alot they love it

Geesh it took Levi months to even TRY millet! LOL They don't seem to be getting fat.