How much spare cable should I leave inside the cabinet?

Enough to allow sensible termination, routing and future maintenance, but not so much that the cabinet becomes long-term storage for surplus cable.

A little service slack is useful. It lets equipment be moved or reterminated without immediately pulling everything tight.

The ideal amount is somewhere between “none” and “whatever was left on the reel”.

Manage the spare length deliberately: support it, keep it away from ventilation openings and avoid large unnecessary loops.