This depends on how you chose to upload the files.
When you've uploaded using our free service, transfers are available for 7 days. After this week they are deleted to make room for new transfers on our servers.
Files sent through a person's Plus Profile (not your own), are available for four weeks instead of seven days.
Plus users can save up to 100GB of transfers they have sent permanently in their online Plus storage until they decide to delete them, or the account expires (which the files will be deleted after 28 days of account expiry). The default expiry period for transfers uploaded by Plus users is 4 weeks, but this can be changed easily.
In 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!