Strictly, it temporarily adds to your strength. It does effect the damage you do on epic battles, but not by a set amount. It all just depends on your current strength and the epic battle you're in.
You will however notice a plunder increase, however minor, due to the static increase in the strength of your actions.
Is it worth it to keep it on for the extra you'd get with the bonus, because of the increase in your average damage?
Yes, I would say so. You will earn more than enough with the spell on than without it, to cover the costs.
Once they are ripe, they are ripe. How long until they rot, depends on how they are stored and their previous conditions.