For fun, I could suggest if Jewel inflicts a lot more pain upon himself, the longer the voodoo effect will last. If it's just small cuts, it ends faster. So maybe 30 seconds-1 minute on average if he just does basic control/injuries, but if Jewel decides to lose his mind and take off an entire arm, maybe it'll leave the other person stuck for longer (until he passes out from blood los-).

Let me know if you're interested, but if you want a flat rate, I'd say a minute to 3 mins is prob ok.

ALSO. As with all mind control powers, if there needs to be an ooc out for whatever reason, enough willpower can free someone from the thing.