If nobody uses your siege weapons, and they just stand there, they will start to break faster than you can repair. To avoid that it's enough if someone clicks on them once in a while. I don't think you even have to shoot or anything. Just click on them, aim around for a second and then you can relax and repair again.
If you exit the siege weapon view and leave it set up, it takes about 10 seconds (for most weapons) for the game to decide you've abandoned it and it starts to degrade at an accelerated rate. The game doesn't care if you're in the process of repairing it. To reset this timer all you have to do is get back on the weapon for a split second. You don't have to do anything and can immediately exit it again and resume repairing it.
If you are talking about after you repair it a few times then all of a sudden it just starts to degrade faster than you can repair it you have to interact with the siege weapon(as if you were going to fire it) then exit it and start repairing again. It's some sort of bug. Happens to me all the time.
I think the rapid deterioration of abandoned siege is fine overall. It's just confusing that repairing siege doesn't reset the abandonment timer, and that the only thing that does is getting on it to fire it. (rams and tents excluded).