How long does it take for a stool softener like Metamucil to take effect?

Not a stool softener. Metamucil is not a softener per se. Rather it adds bulk and can retain (adsorb to be technically correct) fluid which may 'soften' the stool indirectly. The effect is usually within 24 hours. The dose may need to be gradually increased to (a) be effective and (b) minimize gas, cramps etc.