Hi @ELENA IOANNIDOU, thanks for posting today.
If you're currently on a Dropbox Basic and Plus plans, the recovery period would be 30 days, and any files deleted over this period would be unrecoverable, unfortunately.
Upgrading to a different paid plan wouldn't work retroactively, as the files were already permanently deleted from the server.
However, there may be some third-party programs you can use to try to recover the deleted files as they were on your machine at one point.
If you have any further queries about this, feel free to message back.