Manual:$wgWatchlistExpiryMaxDuration

Details
Relative maximum duration for watchlist expiries, [https://www.php.net/datetime.formats.relative as accepted] by PHP's   function. This relates to finite watchlist expiries only. Pages can be watched indefinitely regardless of what this is set to.

This is used to ensure the  table doesn't grow to be too big.

Only has effect if  is true.

Set to   to allow expiries of any duration.