What are lash extensions? Lash extensions are individual false eyelashes that are professionally & individually glued to your natural eyelashes. Each application can take up to 2 hours & can cost between $150-$250. Lash extensions can last from 3 - 4 weeks or fall off when your natural lashes grow and or fall off. You cannot sleep on your face or allow your eyes to touch your pillow with lash extensions. Also when washing your face you must be extra careful.