There is no one standard belt length, but for most airlines the "norm" is somewhere in the 40+ (101cm) inch range.. A typical seat-belt extender is almost universally 24 (60.9cm) inches in length..
What is the standard size of a seat belt?
Seatbelt is woven to international standard widths and is 46 – 49 mm wide; most is 47 mm (1.85 inches). Seatbelt is often described as “two inch,” but is actually not woven to that width (50.4 mm). Being surplus material, this webbing will not carry its original certification.