Many factors contribute to the life of your molds. If your casting with polyester your mold will last longer than urethane. Urethane kicks off considerably more heat and is much harder on your molds, you will notice discoloration very early when using urathane, this is your mold absorbing chemicals and breaking down. You can prolong the life of your molds by using mold releases as well you can clean your molds using acetone.
On top all these factors the type and quality of silicone is important.