Your offer will never expire, but you can always cancel it.
What is W-ETH?
W-ETH (WΞ), which stands for "wrapped Ether", is a cryptocurrency used to only make bids and offers for digital goods on OpenSea. There is a 1:1 exchange between W-ETH (WΞ) and ETH (Ξ), so you can always convert it back and forth anytime.
The first time you convert ETH (Ξ) into W-ETH (WΞ), there's a second transaction needed to approve OpenSea to access your W-ETH when a sale occurs. This only needs to happen once per user.