This article explains how long you can list items for on OpenSea.
Fixed Price Listings
Currently, the longest you can set a fixed price listing is for 6 months.
To select your sale duration, click on the drop-down menu under "Duration" to set your preferred time.
Timed Auction
The longest you can list an item for on a Timed Auction (for both highest bidder and declining price auctions) is 1 week.
