This depends on the amount of steaming you are doing.

In constant steam exposure a silicone diaphragm generally lasts approximately 40 hours while constantly actuating.
A Steam-Grade Viton A diaphragm lasts approximately 200 hours under the same conditions.

Here I've listed our current diaphragm elastomers in order of steam resistance:

Silicone, Silicone Plus, EPDM, EPDM Plus, Viton A (steam grade- coming soon)