Its 48-72 hours for doubling... once you are over 1000 an increase of 60% is considered sufficient. When dealing with numbers around 1000 I would give it 72 personally.
How far along in the pregnancy do you think you should be?
When I was 5 weeks 6 days with HCG levels over 15k my doctor suggested a d&c because no heartbeat could be seen on the scan. I waited another week and everything was there and perfect.
Doctors can sometimes be a bit precipitous and my doctor, in my case, would have ended a perfectly good pregnancy because he was rushing. Babies grow at different rates sometimes.
If your levels decrease or barely increase its not a good sign but if you had a moderate increase even if it didnt double, I wouldnt be so quick to get the shot.
Yes, an ectopic pregnancy can be dangerous but HCG levels before 1000 will never show anything in a scan anyway. HCG levels need to be at at least 1000 to show a gestational sac and often much much higher.