Say I mixed Soilmoist with topsoil for my lawn. Does anyone know how long
this stuff will last?

That would be terribly expensive and probably not that helpful to your lawn.
Soilmoist is designed for potted plants and small specialized flowerbeds. I
would estimate its useful life at about two years, after which you would
probably repot anyway.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)