There is no replacement interval for thermostats, but they're cheap and easy to install.
I can't believe that the dealer or your mechanic didn't replace the thermostat when the head gasket was changed.
Yours is probably still working fine, but it would give me piece of mind knowing that the thermostat was changed at about the same time as the head gasket.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".