There are no set rules regarding how long a short sale can last before being closed out. The lender of the shorted shares can request that the shares be returned by the investor at any time, with minimal notice, but this rarely happens in practice so long as the short seller keeps paying their margin interest.

How long do short positions last?

There is no mandated limit to how long a short position may be held. Short selling involves having a broker who is willing to loan stock with the understanding that they are going to be sold on the open market and replaced at a later date.

What happens if you never close a short?

You're asking what happens if you don't, or can't, and it rises quickly. They will automatically force you to buy to cover it. If that disadvantages you, they really don't care. They will use your brokerage account's assets.