Quick Facts from the IGFA:
A fly fishing leader includes a class tippet and, optionally, a shock tippet. A butt or taper section between the fly line and the class tippet shall also be considered part of the leader and there are no limits on its length, material, or strength.
A class tippet must be made of nonmetallic material and either attached directly to the fly or to the shock tippet if one is used. The class tippet must be at least 15 inches (38.10 cm) long (measured inside connecting knots). With respect to knotless, tapered leaders, the terminal 15 inches (38.10 cm) will also determine tippet class. There is no maximum length limitation.
A shock tippet, not to exceed 12 inches (30.48 cm) in length, may be added to the class tippet and tied to the lure. It can be made of any type of material, and there is no limit on its breaking strength. The shock tippet is measured from the eye of the hook to the single strand of class tippet and includes any knots used to connect the shock tippet to the class tippet. In the case of a tandem hook fly, the shock tippet shall be measured from the eye of the leading hook.