They don't get sour at all. A breeding farm only needs one for 3 or 4 months. We do not breed early, so we do not need one until April and his job is done in June so it is 3 months at the most. There is not much sense in feeding one and listening to one for the other 9 months, especially if you use a full-sized horse.
If you are in this business to make a living, you cannot afford to keep a bunch of dead weight on the feed bill.
I've never had one get sour. They just keep howling and screaming at mares for years. I've had them get so good at their job that I could tie one to the back corner of the feed truck and just drive down past the mare pens. He would squall at every mare and trot along quietly between pens. I have a hard time getting rid of one of those, especially a pony. Most just howl non-stop, day and night so they go to the sale after the breeding season or get gelded.
We always gelded the ones we took to the sale. When my son was little, we let him break them out and ride them through. After that, we went to using registered horses, gelding and training them when we were through with them. A couple sold pretty high.