Depending on the location. Cancer in the head and neck or pelvis may take longer time to recover

You're going to need a few weeks. In 1 month, you should feel pretty good. Cancer can keep responding to radiation therapy for much longer, so effects are still taking place at 8 and even 12 weeks out. 

It depends on how much radiation you received and to what parts of the body it was administered. Generally you should be over the acute effects in another two to three weeks. Make sure you drink enough liquids and eat a healthy diet.