30 feet is what i use, i would never double it up, it does not benefit anything... a doubled over strap still only holds 30 000lbs (if that's it's original rating) because you will have the 2 loops on one end, and a fold on the other, where the fold is, you are putting 30 000lbs on the same amount of material... it will not increase the strength at all, plus, you just halved the amount of stretch it will give, which will put the snap on your recovery point and frame, not the strap...
honestly, when you get out in the bush, 30 feet is not very long... i carry an additional 10 foot 5" strap i use for anchoring to things, it's actually a tree saver, but i use it for hooking my strap to my front 2 tow points (if you use the factory dual tow points on my truck, and pull on only 1, like the guy in the video did, you run a high risk of damaging the cross member they are mounted to, so i pull from both using the tree saver and a shackle)