I am not sure either, but returning the DOT early is not possible going through the chain. Any DOT sent via crowdloan.contribute extrinsic will be locked up until the end of the lease (assuming they win a lease, of course).
DOT / KSM will definitely be returned.
To clarify, it's not just "customary", it's set by the rules of the chain. Anyone who is setting up a crowdloan can indicate the ending point, and after this ending point, anyone can refund the KSM to contributors to unsuccessful crowdloans (although this is usually done by the team that is running the crowdloan, technically anyone can do it).
Kusama auctions (mostly just for scheduling reasons, but also to give engineering teams time to see how the network is reacting before starting a new batch of auctions) have a bit of a gap between groups. Many crowdloans just set the ending point to be the end of the group, when really they should set it a bit earlier if they want to be perfectly optimized.