The maximum amount of length that can generally be added by eyelash extensions is 17 millimeters, according to The Huffington Post. In general, extensions range in length from 6 to 17 millimeters.
Eyelash extensions are a cosmetic procedure in which false lashes made from silk, animal fur or synthetic materials are applied directly to each individual natural eyelash. Lashes are stuck in place using a special adhesive designed for use near the eyes. Eyelash extensions generally need to be reapplied once every three or four weeks. An eyelash extension session usually takes about two hours, though subsequent touch-up sessions may be shorter than the initial session.