Twice the height of the mast plus 10 or 15 feet ought to do it. One easy way to do it is to cut the shackle away and install it on new halyard (knot or splice...your choice), then stich end to end new halyard to cut end of old one, cover the joint tightly with electrical tape, then pull gently on cockpit (or mast base) end of old halyard. New one will follow exactly, go around the mast top sheave, inside mast and out at base then through the line stopper.
Careful not to pull to hard. If it snags, bring it back gently and find out why. The old one is your messenger and you don't want to loose it ! Good luck