Sophiarose, the radiologist and doctors want most of the Octreotide or Lanriotide out of your system so the at least 3 weeks after your last shot is preferred.
The way the scheduling worked for my next Dotatate scan is I'll get the scan on Jan 8th and have my next shot on the 9th. Your doctor decides how often to do them - some have them done every 3 months, some every 6. So the radiology department where you'll have yours done should ask you when was your last injection, and your doctor will determine how often to do them.
Good luck.

My experience over the 22 months I’ve been getting the injections is that they determine the next MRI based on the the results from the previous one, I’ve gone 5 months between them down to 3 months.