depends on the bait and set. location is every thing, i use weasel boxes with a rat trap (just set them last night for a weasel i saw yesterday) baited with meatWeasels are very difficult to trap.
also talked to my friend who is a trapper and recommends for weasels if they won't get caught in a rat trap use a 110 or 120 (legal/if you can) conibear trap in a box. and this guy gets like 200-300 weasels a year on his trap line.