Are we talking about a white hat?
Exactly.
I think it would depend on the situation.
Situation is someone steals your wallet, whitehat knows that your wallet was stolen by a criminal and warns you before the criminal transfers your btc.
I don't think the criminal would wait a single second with transferring the bitcoins after he stole the wallet.
But let's pretend this situation is actually happening, and the hacker is David Hasselhoff... I would give the whitehat probably 5% of the coins in that wallet.