Assume that I have a very very bad user (trying to use nice language) who loves to format his Windows drive once in a week. It's always the same hard drive and the same user.
So, how many times can an OEM-licensed Windows be activated?
As the EULA States
It will activate as many times as you activate it, on one computer, assuming you aren't breaking any other part of the EULA, such as using different hardware or more than one user or processor using the software at one time.