Lots of good points made - I would point out that it is easier to tie a long line shorter than it is to tie a short line longer. Every boater should know how to tie a proper cleat hitch.
Working in a marina, during a storm, we walk the piers and retie any boat that is rubbing. It constantly amazes how many boaters don't know how to tie up their boat, let alone how to tie a cleat hitch or a bowline, for that matter. When the wind is honking and the rain is drenching and a boat is nuzzling up to a piling scraping its paint off, it can be very trying to untie somebody's cleat hitch that's wedged tight because he started his hitch on the wrong side of the cleat. (Yes, there is a wrong side - for those few of you sailnetters who may not know, you always take the line to the far side of the cleat first, else when you take the first loop it wedges and jams.)
Regarding length of springline, ours is almost 60 feet long. (Out boat is 41'.) We tie it to pilings in front of and behind the boat and secure it to a cleat in the middle.
The reason for the extra length is that it makes it possible to single handedly turn the boat around on a face dock if necessary. Simply fasten the extra long springline to the bow or stern - depending, of course, on which way you're going to turn it. Run the line all the way on the outside and back to the dock. Untie the dockline at the end where the springline is secured to the boat and let the wind or current take that end out. (This only works when current or wind is cooperating.) Once the boat is about at a right angle to the dock start pulling the springline back in. In turns, walk the other end of the boat along the pier, and pull your springline (I'm calling it a springline even though it's a bow line or a sternline at this point) until the boat is back parrallel to the pier, then secure all lines normally.
Don't forget to unplug your shorepower cord before starting the whole procedure. (I've never
done this, of course.)