Probably not less than 3.5', otherwise the fish probably see it more than you'd like.

If it's more than 6' then, with a bent rod due to a king on the end of your line, it's tough to LAND the fish.

5' is what I use.

American made ball-bearing swivels spin a LOT better than any type of swivel that I've tested.

When done at the end of the day, consider putting WD-40 inside the swivel, then wipe it off the outside. It will spin better next trip.

Also, if in saltwater, you MUST wash it off real well in fresh water or it won't work next time.

We carry a fresh-water tupperware bucket with a lid in the boat, and toss swivels, flashers, hooks, etc. into it at the end of the day of saltwater fishing...then maybe an hour later we empty the water out and dry off the swivels. We NEVER have problems doing it this way....used to have plenty of problems in the saltwater prior to doing it that way.

Good luck!