How often do you clean hvac vents in each apartment?

How often do you clean hvac vents in each apartment?

Once a year , but we keep them in the office for anyone who wants to change them put themselves in between

Vents and ducts as needed, filters annually unless needed sooner, if used often.

My old company used to require tenants to have filters changed monthly. Helped so much with the upkeep of the hvac units.
We do preventative maintenance quarterly, change filters and clean coils, and vents as needed.
As much preventative as you can possibly do, will help your systems in the long run and help your guys during the summer.

Quarterly we replace filters and clean coils as needed

I may be overthinking the question, and if so, I apologize. If you're referring to the vent covers themselves, this would be done during the turnover process as a part of cleaning. Once occupied, the resident would be responsible to clean them as a part of housekeeping.

If you're referring to the filter, this should be replaced every time it needs it, with the maintenance team scheduling an inspection/replacement a minimum of twice a year. (4 times is best, depending on staffing and equipment.)

If you're referring to the ductwork, this is a highly variable interval. It has no set interval. Care should be taken in older buildings/construction as, over time, the ductwork itself can break down, causing larger issues.