I don't usually put more in than what they eat in a few hours - that might be an easy solution to this? I would remove it after 12-24 hours (less for the mushier veggies that break down easier - you don't want them to pollute your tank).
I mostly use algae wafers and turtle sticks myself. If the snail is up for air I'll give it some turtle sticks for that extra calcium it needs, and every second day or so I put in an algae wafer. If you really want to give them fresh veggies you should make them small and use a feeding dish, so it's easy to remove them and to make sure they don't just get lost in the tank and polute the water. How long to keep them in depends on the veggie. A cooked zuccini slice needs to be weighed down with something, but it dissolves in a good 4 hours to a level of messing with your tank. Meanwhile carrots last much longer, about 8 or so hours before they should be removed. You can also try moving the snail to the food (be very careful picking them up, don't just pull them off the glass, you can kill them doing that! Just gently slide them up until they let go.) to be sure that it actually finds it in the first place.