How long do I need to reveal my hand for when someone uses Scuttlecrab's Deathknell effect?

To answer your question, you need to reveal your hand for as long as it takes for all players present to clearly read and understand the information.

According to Rule 424.3 and the RiftJudge FAQ #4862, when a card effect instructs you to reveal your hand, you present the cards to all players at the table. Once every player has had a reasonable opportunity to understand the revealed information, the reveal action is complete.

Regarding the specific timing of the Scuttle Crab Deathknell effect:

1. Trigger: When Scuttle Crab dies and is sent to the trash, its Deathknell ability triggers and is placed on the chain as a pending item (Rule 808.1.d.2).
2. Resolution: When the Deathknell effect resolves, the chosen opponent must reveal their hand.
3. Duration: The reveal is a temporary state (Rule 424.1.a). You reveal the hand, allow all players to see it, and once the information is understood, the reveal ends.

This is a one-time action triggered by the resolution of the Deathknell ability; it does not require you to keep your hand revealed for the remainder of the turn.