Urriak Uruk
Gaming is fun, and fun is for everyone
They've done three Setting books in 3 Fiscal Quarters now: spacing might not be their concern if there is money to be made.
I personally think this is a unique situation. The Eberron book, as good as it is, had a lot of content already completed and tested from the Wayfinder's Guide. The second book, Wildemount, had been developed for a long time by Mercer himself. Only the third, Theros, is one I think has fallen the traditional development route of Wizard's.
That said, I think we will see at least one setting book every year from now on. I just doubt we'll see a MtG setting every year (every other year, the pattern so far, seems logical).