It depends on your fitness level and goals, but in general you should vary it over time, like every 6-8 weeks. Various "experts" recommend routines like 3x10, 5x5, 4x6, 10x3, 5x8 or pyramid (12,10,8,6,3) or reverse pyramid (3,6,8,10,12)..., but in the end you have to find what works for you (both physically and mentally). The best routine is the one you will do for the long term!