there is no official statement in regards to the recommended number of nights a PM room (or any other room type for that matter) should stay checked in.
Assuming the question is mostly regarding PM accounts used to receive cash and credit card posting from the POS system, then it will be recommended to check them out regularly. This is because these accounts usually receive hundreds of charges per day and if not checked out at some point they can become hard to access and maintain. How regularly they should check them out it will depend on the volume of charges and how the property would prefer to handle the procedure. Personally I would recommend once per day, simply because it's easier for the night auditor to remember to do the task every day instead of once a week or a month, but whatever works best for them is fine.
In case the property doesn't know it, there's a parameter called "Auto Check In for Pseudo Rooms" that when enabled greatly simplifies the process. They just check out the PM (which usually has zero balance because of the auto-balance feature) and a new duplicate PM with the same room number and characteristics is automatically checked in.
If your question was not in regards to POS account PMs, then the limit would be again the number of postings that might accumulate and could potentially affect performance when accessing that account. And in general, PM accounts that don't server a very specific purpose and have a defined life-span are usually not a very good idea.