almost always tie my snells about 60 inches to start with. I use the same length for Lake Erie, Saginaw, Winnebago, or the small lake my Dad lives on in northern Wisconsin. At the end of the leader without the spinner, I tie on a #2 Berkley Ball Bearing swivel.
Rigged like this, I can attach the snell to the same snap (Berkley Cross Lok size 1) that I use for crank baits, and use long leaders with Snap Weights. I can also directly attach the snell to an XPS In-line weight.
The only time I change snell lengths is when I want to run the spinner closer to the in-line XPS weight. Sometimes I run the snell as short as 18 inches.