A long time - couple months at least. Look up some threads on heeling - and what type of heeling do you want to teach? Formal, flashy, obedience heel? Or just informal heel? I'd start by getting the dog used to the basic position (left side), and giving you focus. Once that is rock solid, and the dog understands that the left side is a good place to be, and will give you good focus, start by luring the dog with food and take one step. It is a slow process. Takes time to get a good heel.
I find its easier to teach Sabo to heel of leash. Treat in hand, covered by my thumb, four fingers down, put it in front of his nose. When he wants it, start walking forward slowly and tell him "heel". Take three or four steps, stop, pull your hand up so your arm is 90 degrees (bed at the elbow) and lure him into a sit- praise and treat. Repeat.