There are 4 degrees of false eyelash lengths: short, medium, long, and very long

1. Shorter than 8mm: short (just like the natural eyelash)

2.9-12mm: medium (mascara)

3.13-14mm: long 

4.15mm and above: very long

Certainly, the above is a general rule. Depending on your eyes, the lengths can look shorter or longer than others with the same length.