My mother had a cortisone shot 4 weeks ago and is still in severe pain. She is also extremely week still her doctor keeps telling us to give it time I am just not sure how much time to give it.
I can only relate my own experience for you.
My shoulder was hurting so bad that I couldn't sleep at night so I finally went to the ER at 9:30 am. They gave me a shot that had a painkiller in it along with the cortisone. The shot wasn't bad at all and when I left at about noon I was able to reach out to close the truck door with that arm with no pain at all. I had not been able to do that for several weeks. Around 3:00 pm the shoulder began to ache a bit and by 6:00 pm it was hurting many times worse than the original pain I went in to get fixed. I was seriously considering going back to the ER but around 9:00 pm it seemed to be getting better. By 11:00 pm when I went to bed it was quite tolerable and I was able to sleep through the night for the first time in weeks.
After that the only pain was the original pain which kept getting better until it was just a memory in a few weeks.
I complained to my doctor's office girl that they didn't tell me it was going to hurt like HELL for a few hours soon after the shot and she laughed because someone else had just had the same experience. We surmised the reason was they figured most people would back out on getting it if they were honest about how bad it was going to hurt.
However by 4 weeks I was having very little if any discomfort. It's hard to say exactly how long because my shoulder had been hurting for years and continually getting worse. So any serious reduction in the pain would feel like it was almost gone.
It was worth it though because it's been about seven years now and it still works fine.