2 feet (unless you've got a peg leg) Bad ooomp.... pahhh

Seriously though, typical cab about 6 feet high, 3 ft deep, little less than 3 ft wide
So that's 6+6+3+3+3=~21
if you go all the way up the front, down to the back on the top, then across on the CP.
If you want to put more tmold on (trim the whole CP, or run tmold down the sides of the CP as well) or if you're cp is extended like a 4player, or your putting lit cutouts on the sides, you'll need more.
I'd guess 30ft would likely be plenty, and 40ft would be way more than you'd need, but you'd be safe.
Sometimes you can get remnants for a little cheaper too. I bought a bunch of black leather texture tmold that way, came in 2 20ft "scraps".