From my experience, it never deleted my files unless something like an server accident.
How long does Mediafire store files?
For MediaPro subscribers, your uploaded files will be stored forever as long as your Pro account is active. For free accounts, there is currently no time limit on how long uploaded files will be stored as long as you access your account (i.e. login to your account) and your files are being accessed (i.e. downloaded) on a regular basis. Every account is different and different accounts may become idle at different times. As a file sharing service, our goal is to flush out users who are through using MediaFire and their data is collecting dust.
Users receive at least 3 notification emails that files are pending for deletion before any deletion actually occurs. Logging into your account is sufficient to prevent deletions.