This is very dependent on the part design and the materials involved. Silicone molds are typically used to cast resin parts. Most resins produce exotherm (get hot) as they cure and this is what damages or wears out the mold. Thin spots in the rubber mold will dry out (burn out) quicker than thick spots. That being said, most resin casting silicone molds will last anywhere from 50 to 300 cycles.
Polyurethane molds are typically used for casting concrete products like veneer stone or pavers. Although concrete does produce exotherm, it does not get nearly as hot as resins. Casting concrete in urethane molds normally requires the use of a concrete release agent. Some of these can be harmful to the rubber and diminish the mold life. The life of the mold also depends on the part geometry and method of demolding - manual or automated. That being said, you can expect to get anywhere from 250 to 1000 + concrete castings out of a polyurethane mold.