My philosophy is to always be thinking about the next maintenance item that can be tackled. When a renter moves out & there are no obvious maintenance items that need to be done, I choose something to upgrade or modernize. My husband disagrees with this attitude, but this is a rule followed by other business owners I've worked with. For example, when an airplane company purchases an airplane, they attach an expected life to than plane. At the end of that time period, the company is supposed to replace the plane whether or not it is still operating fine. The reason for this rule is that it allows the company to plan for the expense & prevents many airplanes from needing to be replaced all at once.