My speculation is around cognitive function. I suspect it is already a known issue in the DNC and plans are being made to invoke the 25th Amendment at some point should they get elected.
I say anything beyond 31 days after inauguration is in play. That is the current shortest Presidential tenure, held by William Henry Harrison who caught pneumonia and died 31 days after Inauguration Day. Past that, Joe doesn't go into the record books.
*MySmiley*
"Bustin' makes me feel good!"
Ghostbusters, by Ray Parker Jr.