This attack's Base Power is anything from 1 up to 120 making it a fairly powerful attack to be used. The means of determining the Base Power however is set based on the Opponent's Hit Points. The higher its hit points the better. This does not refer to the Base Stats for the Hit Points but rather the opponent's HP relative to its maximum at the current point in time.
With this in mind, a simple equation is made to determine the base power of the attack
Base Power = 120 * (Opponent's Current HP / Opponent's Max HP)
So for example if we were battling a Max IV'd, Max EV'd Torterra at Level 100 and it had just over half its HP remaining, it would be as such
Base Power = 120 * (215 / 394) = 60.03 (2 d.p.)
This shows that the best time to use Wring Out & Crush Grip would be at the start of a battle with any Pokémon, whether it has just been switched in or the battle is just beginning, in order to get the maximum Base Power. As the battle progresses, the power behind Wring Out & Crush Grip diminshes