How much artery blockage causes pvd pain?

Everyone's different. The pain of pad is from tissue (muscles, skin, nerves, etc) not getting enough blood. An elite athlete might have symptoms from a single 50% plaque. A sedentary person might not notice a complete blockage or multiple partial blockages. Rest pain is usually from severe plaques at multiple levels. In general, the body compensates for artery plaques for a long time before pain occurs.
Depends. Interestingly it is not only the amount of blockage which causes pain when patients have pvd, but also how quickly the blockage develops. If it occurs slowly over years, many people don't necessarily have dramatic symptoms since the body has time to develop colaterals. If a blockage occurs abruptly (overnight) then even a short blockage can cause dramatic painful symptoms.