How much time after a blood transfusion should you wait to be active?

It depend. It depends what you mean by being active, for example playing sport, or up and around. For the sport , i think one week because after you give you loose volume of your blood and it needs some time to adjust for that loss. For minor activity if do not feel dizzy or weak, do it as far you feel okay.
Until feeling better. If you need a blood transfusion, you have anemia and you are feeling weak at a hospital. Until feeling better and if your doctor recommends that you may continue your daily activities, then you may do so. Regarding blood donation, you may resume normal activities 30 mins post donation, and wait 24-48 hours for more intense activities like contact sports.