What rules or regulations should a police department abide by in regards to keeping their radar equipment maintained and calibrated?

There are no hard and fast rules. If you have been stopped for speeding and believe it was in error, you can, of course, attempt (if you go to trial) to show that the equipment was defective, not maintained property, not used properly. This, however, could be very difficult and expensive, as it would involve getting manufacturer-data and maintenance recommendations, having the specific radar equipment analyzed by a professional engineer, going over the training and procedures of the police, etc. I believe that very few people have ever won by challenging the radar gun.

Gettinig back to your question: it needs to be accurate and used correctly. IF it can be shown that was not the case, then that could throw out the ticket. If it is shown to be accurate, no one will much care about the exact maintenance schedule.