Blank/Erased VHS Tapes

[mwplefty]

I was arguing with my father about how unlikely it was for the magnetic tapes to erase after a certain period of time. Does anyone know roughly how long it takes VHS tapes to automatically erase if it's played on an occasional basis? He said 20 years. I thought much longer. He's been paranoid about his 20-25 year old VHS tapes and quickly converted them to DVD. Is he worrying too soon? Should he even be worried at all?