There is no standard thickness. Depends totally on the application.
Precision machined parts need thin gaskets, sometimes used as shims.
Cast parts, like carb flanges, thermostat housings, etc need thicker gaskets.
Sheet metal, oil pans, valve covers, use thick cork or rubber.
Also depends on what you are sealing, the pressure, temperature, etc.
Gasket material does have a shelf life. It gets old and dry and very difficult to deal with. I usually buy what I need when I need it.