3-5...So, let's say 4!
Do the 2 adventure "paths" - though I, personally, do not use them. They seem to want to produce their adventures this way. A mistake, I think, but if they were to go tback to the older model of smaller stand-alone or [possibly] loosely connectable "modules" of a certain level range, then they could make this 4 instead of 2...which, then, ups the total count to 6. BUT, let's assume, for now, the adventure paths stay. So 2 per year on those. 1 per 6 months.
In between those, so you have a release of SOMEthing every 3 months for the next...2-3 (I could see up to 5) years...or, maybe if they're very savvy get them ALL out pre-holiday season and coast into summer with only 1 adventure path to worry about...BUT, let's say, for now, in between the adventure paths, so there's something to annouce/release every 3 months, year 'round.
Those "in between" supplements can be...whatever. I don't care. I would probably, ideally, suggest making 1 per year a campaign setting book.
The other being a content/character options style book: be that a Psionics Manual, expanded Spell Tome, a Player Races of the Multiverse Compleat, a "Waterborne/Frostborne/Desertborne Adventures" [with multiple race/class options and optional/expanded rules for running campaigns in XYZ locales, possibly with a minor/small adventure included?]...whatever. Ideally, whatever it is has material that is both for player and DM use in it. So, instead of an Unearthed Arcana book itself, we end up with multiple "mini-UA's" on a variety of topics. But not a Setting specific book.
So you'd get, every 3 months throughout the year, hypothetically: [Jan.] Adventure Path year 1.I, [April] Setting Guide, [July] AP year 1. II, [Oct.] Non-setting guide Player & DM useful manual of some topic...[Jan.] Adventure Path year 2.I, etc etc...
With UA articles presenting bits n' pieces of what they're working on and seeking feedback on same, 1 per month.
Following this model, they could be producing a steady stream of material, covering settings some people are clamoring for and player options other people are clamoring for. And they're still only having to worry about producing 1 bound book, likely not even half the size of the PHB or DMG, every 3 months.
In the immortal words of Trathenigan the Conjurer, "Easy peasy lemon squeezy."