Depends on the state (age, bad, very bad, unusable) of the capacitor, the current available to reform it and possibly other parameters.
In my experience if the capacitor can be reformed at all, one hour is sufficient. If after one hour it could not be revived, it is beyond hope.
Usually I prefer to replace bad (electrolytic) capacitors. Even if you can reform them, they'll have lost capacity. You never know for sure how that's going to affect the circuit.