It's probably harder than that! It is from the hardware store and it is quote, "once-cured it can be shaped by sanding and filing".

Other than this stuff, the best I could find was 60A. I debated that since I thought some flexibility might be best for NVH, but then I said screw it and went for the hard stuff just to make sure I don't have to do it again.

I cleaned out the mount last night and it was a beeeaaootch!
The urethane (flexible type) that I put in before had filled in the center of the two rubber disks and it was sooo tough to pull out! It made what should have been short work with a drill of about 5 minutes to get the stock rubber disc out to an hour process! And it was still flexible, like a damned rubber band. So I didn't achieve anything the last time but to make the stock rubber stuff more secure with no added stiffness.
All it probably did was preserve the stock rubber longer. Oh well, its out now and I'll be casting the new mount in an hour or so.

How long do I let this stuff cure for?

Former owner of '99 CSVT - Silver #222/2760 356/334 wHP/TQ at 10psi on pump gas! See My Mods '05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red '06 Mazda5 Touring, 5spd,MTX, Black