How and when do co-ops adjust maint costs?

5% planning
95% having a gun held to their head.

Right. "Gee, given how this year's looking, better get the budget done and spring the bad news...."

My building jacks it up effective in January. One year we had a fuel surcharge that started in the summer, I think.

30yrs nailed it - coop boards typically have no clue what they're doing and for the most part listen to the managing agent. When the managing agent says finances are looking shaky, maybe time for a bump, the board asks, "How much?"

How: maintenance fees are voted upon by the board of directors generally at any such time as the board deems it appropriate to take up the question.
When: most boards voting increases make them effective for the January maintenance bills that shareholders receive. In recent years, as stated above, some have added midyear surcharges. If the city changes tax laws, etc mid-year, then coop boards may also act accordingly. But these are the general answers