Ok who can tell me the rule of thumb      of how long a pr snake skins needs to be  

Two 25 inch skins shoujld do it.        Just measure the actual limb length you want to cover.  Usually, you'll run the skin under the leather grip on a longbow and stop at the base of the tip overlays on glass laminated bows.  Look for skins that are several inches longer so you have some flexibility in positioning them on the limbs.  Of course, it should go without saying that the skins need to be wide enough their entire length to cover the limbs.