Scan results waiting time

Hello..I've been a cancer patient for almost 11 years now. I've never been in remission but manage to live a fairly normal life. 

My question is how long is reasonable to wait for scan results? I used to get them the same day but over the last couple of years I've waited for anything up to 3 weeks..I don't have an appointment to get the results I normally just get an email or a call. 

How long do others wait for results?