Projects bidding for a Polkadot parachain can aim for a 2-year lease (most sources will show 96 weeks), which is the maximum.
It’s also possible that a project wants to extend its 2-year lease. In that case, once the lease is over, it will have to enter a new auction and potentially face other competitors aiming for the same period.
Slots are technically not sold at the auction but leased against collateral locked in DOT. The bids are returned to their respective owners at the end of the lease period. Furthermore, the auction mechanism enables bidders to gather additional funds to bid for access to a parachain if they manage to mobilize holders of DOT to bond some of their holdings in support of a project. Such supporters incur only an opportunity cost when bonding their DOT. During the entire auction process and the lease period that follows, the bidder never gains custody over the tokens of such supporters.