TLDR: The answer? 7 days! If that's too short, sign up to our paid service, WeTransfer Pro/Premium, and take more control of your transfers.
When uploading with the free service
When sending transfers through the free service (at https://wetransfer.com) they are available for 7 days. After which they expire and are removed from our servers. We don't keep a hidden stash of transfers so once they are deleted, it is no longer possible to download them.
In the case you're using the free service, please make sure you save your transfers to your hard disk, because once we've removed a file, there's no going back!
When uploading by visiting a WeTransfer Pro or WeTransfer Premium account
Files sent through someone else's WeTransfer Pro or WeTransfer Premium page (not your own), are available for four weeks instead of seven days, and will be saved in the WeTransfer Pro or WeTransfer Premium account's transfer overview. This is just one of the many features of WeTransfer Pro or WeTransfer Premium - you can learn more about what else WeTransfer Pro can do right here or for WeTransfer Premium just here.
The same rules apply as above: once the transfer expires, it really is gone forever.
When uploading using a WeTransfer Pro or WeTransfer Premium account
Files uploaded in WeTransfer Pro or WeTransfer Premium accounts have been sent permanently to their online WeTransfer Pro or WeTransfer Premium storage until they decide to delete them, or the account expires (in which case the files will be deleted after 28 days of account expiry). The default expiry period for transfers uploaded via WeTransfer Pro and WeTransfer Premium is 4 weeks, but this can be changed easily to ensure your files stay online for however long you like!
Don't have WeTransfer Pro or WeTransfer Premium yet? You're missing out! Just sign up here.