I think what WhatsApp does is, it checks the file on the device of the sender every 30 days. If the file exists, it renews the file on the cloud for another 30 days. If the file is deleted from the device, then it gets deleted from the cloud after 30 days.
Here 30 days is hypothetical. So if you want to download after 2 months, then it will depend on the sender, if he/she have it on their device or not.