Some publishers pay royalties quarterly and some every six months and the sad reality of the publishing industry is the uncertainty about anything coming at all. Not only do people have to buy books, but the publisher has to receive the monies from distributors and the distributors from booksellers, a chain of events that slows the process to a crawl. One other imponderable, is the survival of the publisher themselves and I've been caught by the demise of two so far. (it's a risky business for small publishers)
Coincidentally, this month all my royalties are due and the first one arrived today, a small one. No matter, I know that publisher has survived another quarter. I must now wait to see if the others have been as lucky.
It's a great life.