Pretty much the standard length is 3-4' If attached to your crotch strap or somewhere low on a BCD this is generally fine. You can inflate yourself up a bit to keep off the reef/rocks
You can of course go longer but the balance here is, if your hook comes out and you're too high off the reef, then you're away like Mary Poppins. The higher you are, means the more exposed you are to the current too. So if you're just wanting to hook on and watch the fishes then 3-4' is fine.
All that said alongside my standard hook, on some dives I carry an extension which I can clip on (either to the hook or the end of the reef hook cord It adds another 8m of line (25') so I can fly well off the reef and into the blue. But I have to be damn sure my hook is well in before I deploy it
It's also a PITA to chain knot and tidy up at the end of a dive
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