If you are keeping ewe lambs on as breeders, and not tupping them as hoggs, then yes two years. After that he'd be on his own daughters.
Depending on the breed, you may be able to buy an 'aged tup' - one that someone is selling on for the very reason he'd be on his own daughters on their spot. These are usually good buys; if he's worked two or more seasons then he's been leaving good lambs

, and aged tups are generally not too expensive so you can often get a genetically very good one for not too much money.
Another tactic is to buy a good tup lamb, use him once then into the fat with him. If you're breeding for pleasure you may well find a perfectly good enough tup lamb in the store or fat ring so only pay fat or store price for him.
If you're breeding for sale and/or showing, then you may want or need to buy a tup or tup lamb bred for breeding and with a pedigree, breed registration, and so on. He'll cost more so then you may want to try to sell him on as an 'aged tup' once he's done his two seasons with you. Or yes, find another small breeder of the same breed and see if you can't do a bit of swapping
