I'm certain this won't answer your question correctly, but just to be on the safe side I'd stay at least once every 12 month period.

Before I actually started paying attention to these programs very recently, I had three SPG accounts and each time lost points because there were some 12 month periods where I never stayed at a W. I had the same thing happen with Hilton, I think twice. Now I ensure I stay at least once every 12 months and have not lost points since !