Who inspects commercial aircraft seats for defects (and when)?
Apart from what ymb1 and Zach Lipton mentioned, the passenger and flight crew seats are thoroughly inspected during the scheduled maintenance of the aircraft, in which the aircraft can be grounded for several days up to some weeks. Depending on how extensive is the maintenance check, the following items can also be inspected amongst others:


Condition of seat floor attachment fittings for tightness.
Reclining function, except those before and after the emergency exits (these must not recline at all).z
Removal of the seat cushions in order to inspect the seat structure.
Periodic replacement of all seat covers (washed or new).
Seat belts for condition, operation and proper attachment to the seat.
Life vests for presence, condition and expiration date.


Sometimes, when the repair is beyond the capability or the approval of the maintenance center it can be sent to a specialized seat overhauling center.

For more information and fun you can check the British Airways B747 D-check video