The ladybugs will stay in your garden as long as there is food for them. in the form of aphids and other critters. When they run out of food, they'll move away, probably to your neighbors' yards, but if you get aphids again, they'll come back.
You can keep them in the neighborhood by putting up places for them to winter over, such as unused bat or butterfly houses, or even a vacant birdhouse. I never released any in my yard, but I had a lot of aphids, so the ladybugs came. They would winter over in a hollow branch in my big tulip tree.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams