Nothing is kept to a strict schedule (but maybe that should change). We do username changes twice a year, tournaments every few months, promotions roughly every year, and maybe other things that I'm forgetting.
If we kept username changes to a perfect schedule as of today, then you can expect the next thread to go up on exactly the 21st of October, sixth months after the last one. Then, you could also expect username changes to go up on 21 April and 21 October every year... though history says we're unlikely to keep the pace. (I think Pokemaster like to do them on weekends either way.)
Not sure if this answer is good enough to attach to the useful posts list, but this tag might be. There's info in there about how username changes work and how staff work, and I plan to add stuff about tournaments too once we've got this figured out. (I'll also properly state what's happening with that tag, once I've finished writing the last few threads.)