A longbow attached to an orc weighs a lot more than 20 lbs. Wresting it away from him should require opposed strength checks... As someone pointed out, 20 lbs = strength 2. That's a -4 to strength checks. A check against Will, as some suggested, makes no sense, because there's no way you can fail to notice a glowing hand trying to take your longbow. A check against reflex doesn't make much sense, because this isn't tricking them into dropping their bow or hitting the bow hard enough to make them drop it, it's opposing their own strength to wrestle it away. Some have proposed using int - but that doesn't make sense because there's a hard limit of 20 lbs (strength 2) on the ability of the hand no matter how smart you are.
Given that it's a move action to get it to the creature, and a minor to try to pick up 20 lbs, I'd say this isn't a fair trade off. Even an orc drude is capable of lifting 160 lbs, with a +3 strength modifier, so the mage hand is virtually guaranteed not to win this tug of war.
Even if you allowed an opposed strength check, it's unlikely. On an average check, the orc will score a 13 vs the mage hands 6. If the mage hand rolled a 15, and the orc rolled an 8, that's still an 11 for the mage hand and an 11 for the orc. Barely a success - his hands were slippery, perhaps wasn't expecting this to happen despite seeing the hand approaching, and the mage hand wins.
THEN it still has to move away with the bow, another move action. If it doesn't move, it remains adjacent and the orc can grab the bow back as a minor (since we ruled that's a minor action for the mage hand to work properly) or pull out his axe and chase after the meddling wizard.
Either way, this was a ridiculous waste of time on the wizard's behalf. (since at range, several move actions to get to the orc, a minor to grab the bow, and at least one move action to get it away from the orc.