Manual:$wgWatchlistExpiryMaxDuration

Details
Relative maximum duration for watchlist expiries, as accepted by strtotime. 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 watchlist_expiry table doesn't grow to be too big.

Only has effect if $wgWatchlistExpiry is true.

Set to null to allow expiries of any duration.