If there are minnow around a few hours should be enough. The trouble with leaving out for days is turtles, salamanders getting in and eating your bait. I keep moving around until I start finding bait then saturate the area for a day or so. This time of year think small pounds, they warm up faster that moving water. Knowing were you live I would try the fishing lake but don't leave your traps, someone will steal them. Burlington Lake is good as well. I got about a 1000 minnows in four traps in four hours Thursday last week.