My left leg is swollen from the thigh down to the leg from a blood clot cause I'm on blood thinner. How long would swelling and pain last?

Blood clots. Depending on how bad your clot was the leg could be larger forever. Though some of the swelling will go down. I don't know how long you have had this but it could be months before it improves. Your doctor can recommend pain meds based on what medication you take.