At the minimum, each time you apply the critical patches or as needed.
If you are not patching, that is a different issue... :-)
It is not as prophylactic as it used to be, and that is why you don't see written guidelines any longer.
As long as none of your installed components have a memory leak or you are not using RDS to connect to the machines over and over again, it should not be that necessary.
Only with monitoring will you truly know the requirements for your case.
But always design the solution knowing that you will need to reboot at some point for some reason - this should always be the case, regardless of the OS.
Brian Ehlert
http://ITProctology.blogspot.com
Learn. Apply. Repeat.
