Ideally, eyelash extensions should last for the life of your natural lashes, which is usually around 4-6 weeks. 

Keep in mind however, we can all lose up to 5 natural lashes each day as mature lashes fall out and new ones grow. 

The longevity of your eyelash extensions are also influenced by many factors including but not limited to; lifestyle, use of make up products, cleansing routine, skin type, and lash growth cycle. 

By getting an infill every 2-4 weeks, you can maintain the fullness of your lashes indefinitely.