Having some pain in your arm after you have a tetanus shot is normal and the arm will hurt for a few days to as long as 4 weeks in some cases.
After a few weeks though the pain should not be as bad and if it is or the pain gets worse then see a doctor about it.
The pain should not last longer than a month but sometimes people have reported that they had pain in their arm for as long as 3 months after the tetanus shot.
The last time I had a tetanus shot which was around 5 years ago now the pain in my arm lasted for 2 weeks before it went away.
So you might have the pain for a few weeks and then again you might have it longer.