SpudFiles

How could I take a tank (about the size of a paintball tank) with a pressure of 300 psi and regulate it to 60 psi? How long would a cloud last with that?

You could probably regulate the pressure down to 60 psi with a regulator.
The length of time a cloud would last on that would depend on the caliber, because the larger the caliber of the barrel, the faster air can flow out.

It's a 6mm barrel

You can expect about.....5 seconds max.