1. Anytime I suspect a virus infection (hasn't happened for maybe 2 years.)
2. When I want the latest Windows updates (about every 2-3 months.)
3. When I feel like it to unclutter my system (conveniently, every 2-3 months.)
4. When I install something I don't want (I don't want Turbotax on my system after tax time.)
So basically I restore every 2-3 months.