You should never continue to pick a lock that you have already beaten several times. If you pick the same lock enough times in the same way you’ll eventually memorize the process of picking that particular lock. From there on, picking that lock becomes more about recalling a series of vague events rather than relying on your skill.
As a matter in fact, continuing to pick the same lock over and over can actually put you a few step back in progressing your skill. As they say — if you don’t use it, you lose it.
So continuing to challenge yourself is crucial for improving but the problem is, this can get expensive. How can we be expected to buy a new practice lock every time we conquer one? Beating a lock should be a time for excitement and joy, not a time to feel sorrow for our wallets.