In theory, never. In the real world, it depends on the applications running on the system. Poorly written applications that result in memory fragmentation and locking memory segments from being recovered for use of the memory pool can cause the system to become sluggush or stop responding completely. A highly framented hard drive can cause a fragmented page file with similar results. I have found that at the minimum, a once a month reboot is desireable. In the case of some applications, every week or every other week may be advisable.
