So 105 votes so far gives us a nice cross-section of EN World. So far 15% are happy with the current output or less, and 85% want more. Most want just a bit more, but weighting the percentages it seems like the "ideal" output for most would be in the 4-6 products per year range.
One new product every two months sounds good. How about this: three "major" products and three "minor" ones. Major products would generally be hardcovers or box sets, and include campaign settings, big splat or theme books. Minor products would be under 100 pages, generally softcover, and anything form one-shot modules to short adventure compilations, or setting expansions, gazetteers, etc. Each quarter would see one major and one minor release, with one month off. So there would be a new product every other month, with a major release every four months or so. Realistically, though, it wouldn't necessarily be evenly spread as you want a major release in August for GenCon, and then one in late November for the holiday season (early December might be a bit too late).
Alternate:
Another approach would be to think of each of the main product types and then do one really well each year. Like so:
Campaign book - e.g. Forgotten Realms, Greyhawk, Dark Sun, etc
Splat/Theme book - e.g. Unearthed Arcana, Psionics, Manual of the Planes, Underdark, etc
Creature/Foe book - e.g. Fiend Folio, Monster Manual 2, Draconomicon, Undead, Deities & Demigods, etc
Story Arc book - e.g. Elemental Evil, etc
Then each year could be centered around a theme, with products all linked thematically. For example:
2016: Underdark
Campaign book: Greyhawk
Splat/Theme: Underdark Adventures
Creature/foe book: Fiend Folio
Story Arc: Giants-Kuo-Toa-Drow type adventure; and/or "Greyhawk classics" in 5E form
2017: Psionics
Campaign book: Dark Sun
Splat/Theme: Psionics Handbook
Creature/foe book: MM2 - Psionic monsters/Creatures of Dark Sun
Story Arc: Dark Sun campaign
2018: The Planes
Campaign book: Planescape/Spelljammer
Splat/theme: Manual of the Planes
Creature/foe book: Deities & Demigods (but less focused on gods, more on extra-planar creatures...Devas & Demons?)
Story Arc: Something big and epic - Hellbound? Dead Gods? Etc.
2019: Dragons
Campaign book: Dragonlance
Splat/theme: Dragon Quests (Dragon-based campaigns)
Creature/foe book: Draconomicon
Story Arc: War of the Lance, or some mega Dragon Quest
2020: Undead
Campaign book: Shadowfell
Splat/theme: Undead Adventures
Creature/foe book: MM3: Undead!
Story Arc: Ravenloft
2021: War
Campaign book: Birthright
Splat/theme: Kingdom Building/Conquests/Wars
Creature/foe book: ?
Story Arc: Some epic war
I don't know, something like that. You may notice that there is no Forgotten Realms. There are only four products listed above; maybe two products could be dedicated to the Realms as the "default" D&D setting - a setting expansion book and a story arc.
Again, I'm just throwing some ideas out there for fun.