Did A LOT of this over the summer this year, and was amazed how a fish you would consider "spent" swam away strong. Our Berkley digital scale doesn't work anymore because of this, but it saved a lot of Kings and Steelies in dead summer. Vince is correct when he says you will know when they are ready. I watched a buddy do this and was amazed how well you could revive a fish, while still trolling, with a little time well spent.
So, I guess moral of the story is these fish can be revived successfully, and don't use a digital boga scale to do it! LOL