When should you prune a lily?

I bought asiatic lilies at Home Depot when they were on clearance (and looking very bad), but for 50 cents, who could go wrong? The first year I planted them, they grew about a foot tall. I cut them back to within several inches before fall. The next year, they grew about 3 feet, so I did the same. This is their 3rd year and they are about 5 feet tall and bloom profusely. They have multiplied from the original 4 purchased to about 25. I have never divided them up. Last year I purchased 4 more and put in planters on my porch. I cut them back after they quit blooming, so we will see how they do. I plan to transplant these into the garden in the spring. My advice to anyone planting them, is to expect them to get bigger and bigger each year.