It depends - not what you wanted to hear right, but the answer is probably what you expect...
The areas of a court that require regular maintenance are the walls and floors. Keep an eye out for holes and cracks in the walls, especially if they are getting bigger and any broken or ‘dead’ boards. Try to keep a record of dead floor boards if you are not replacing the floor to let your contractor know where they are - they can occasionally be missed. These things will need to be repaired when you have the pros in to carry out the maintenance.
So aside from major cracks, holes and board breakages, how often should you have your courts done up?
If you have the money to spend and you want to keep your courts in top shape then annually would be best and there are some things that should be done more than others, such as having the walls cleaned. However, we should generally consider the amount of use the courts get and also how they are used. For example, if they are used for junior coaching, the floors are likely to require sanding more often. If one court is used more than the others and is accumulating darker patches on the side walls than the other courts then that is a good sign it needs to be refurbished.
Of course, there are courts around that haven't been touched for years and they are still standing but do you ever hear somebody say 'I love playing on those courts'? Didn't think so.
The general rule is, the better facilities you have, the more members you will attract. In which case a regular investment in keeping your courts looking and playing great could attract more members.
Speculate to accumulate!