depends on the density and size of the vegetables. cut veggies in slices wedges or cubes of similar size...anywhere from 30 min to an hour at that temp. But uniform size is important so they all cook at the same time. THings like tomatoes take an hour at 350F, asparagus takes 15 to 20 min at 400F if it is large, etc. So if you are puting carrots potatoes onions and parsnips around a leg of lamb, go for wedges 1/6 of the whole veg, or uniform bias cuts on the carrot and parsnip, about 45 min should be enough, but be ready to go for an hour.