Lash extensions will last between 3-5 weeks, depending on your home care and retention rate.
Fill ins are every 2-3 weeks. Lashes are similar to nails, they grow out and fall out. A fill in is a sevice to refresh your lashes by removing old, grown out extensions, and relashing natural lashes without an extension that have grown back in.
Like any beauty service, if done incorrectly, yes, it can cause damage. However, I value your lash health. I try to get my clients as close to their lash goal, as long as it is not damaging the natural lash.
Every lash is on its own growth cycle. When the cycle ends, the natural lash detaches and falls out, then rests before regrowth. The extension falls out with the natural lash when it is ready. On average, you loose 3-4 lashes a day, however you may notice this happening since there is a extension attached to it. This is a natural function of the hair growth cycle.
No, as long as you are coming in consitantly, you do not need to remove and get a new set. Because of the natural growth cycle, when I perform a fill, I am simply placing a fresh extension on the lashes that have fallen out and regrown back.
Every set is customizable, and you may change your fill in style as often as you would like.