How long should you wait before jumping a horse after a few months off? The horse in question has had work here and there.

It takes 2-3 months of constant work for horses to gain all of their muscles back. If you jump a horse too high with not enough muscles it can lead to injuries and soundness issues. I work flat work a horse for atleast 1 month before any jumps over 18'.

Hi Aly,
It depends on your horse's shape. If he recovers fast, you can start doing small jumps two weeks after you start schooling him again. The most important is to listen to your horse and you will feel him being ready or not.
Start gently with short jumping session.
Good luck!