Standard wisdom dictates that the tip of your tie should hit at the center of the beltline on your trousers. Any longer and you risk looking like Steve Urkel. Any shorter and you could be mistaken for one of the Three Stooges.
Ready to wear ties tend to be fifty-five to sixty inches in length. For the average male most ready to wear ties are the appropriate length to conform to the stated convention.
If you are either very short or very tall, or if you wear high-waisted trousers, then you may need to invest in bespoke ties in order to obtain the proper length. If you are in need of such services, Sam Hober is a much-praised purveyor of bespoke ties.