Unlike photos, video footage is heavy, often hundreds of GB for a single project. A vague retention policy quickly turns into hard drives spilling over or, the other way around, footage deleted too soon on the day a client asks for a re-export.
Separate the final export from the raw footage
The final export (the delivered edit) is relatively light and cheap to keep for a long time. The raw footage is what eats most of the space: that's what your deletion policy should target, not the finished exports.
A reasonable window for footage, written into the contract
A common benchmark: keep the raw footage for a few months after final delivery, long enough for a client to reasonably ask for an adjustment or a second edit. Past that, you purge, unless there's a specific agreement (a long-term retention contract, usually billed on top).
The delivery isn't your archive
The link you send the client to view or download their edit doesn't need to stay live as long as your internal footage. Let it expire once the download is done, and keep your archiving policy strictly separate from the delivery.
On xFer, you set the expiry of each delivery independently from your own footage retention policy, which stays on your own storage.
In short
Keep finished exports longer, purge raw footage after a few months unless agreed otherwise, and never tie the lifespan of your client delivery to that of your internal archive.