Why the calendar beats the mirror
Natural lashes are on a six to eight week growth cycle and you shed two to five per eye per day whether you have extensions or not. That is not lash loss, it is a lash cycle, and it was happening before you ever sat in a chair.
The arithmetic is simple. At two weeks, roughly seventy per cent of your set is still there and a fill is a fill. At three weeks it is about half. Past that, rebuilding from what remains takes as long as a full set, so it is priced as one.
Nobody notices at week two. Everybody notices at week three. The five days between those two facts is the whole reason to book on the calendar.
What a fill actually is
Not adding lashes to gaps. Every extension that has grown out past a certain point comes off first, because an extension on a lash that has travelled two millimetres out from the lid is now sitting at the wrong angle and is what makes a three-week-old set look messy rather than sparse.
Then the line is cleaned, and then the map is rebuilt on whatever natural lashes are currently long enough. This is why a good fill looks like a fresh set rather than like a patched one.
Coming from somewhere else
Send a photograph first, close up, in daylight, eyes closed. Iris will tell you honestly whether it is fillable. If the isolation is poor, filling on top of bonded lashes compounds the damage, and the right answer is a removal and a fresh start even though it costs more that day.