NaillyHere is the rule most techs use, stated plainly: book a fill when the grow-out gap is under about a third of the nail, nothing is lifting, and you are keeping the same system and roughly the same look. Book a new set when there is lifting or breakage, when you are changing shape or switching systems — gel to acrylic, gel-X to BIAB — or when the current set has already survived two or three fills. A fill typically costs $40–65 and takes about an hour; a fresh set runs $55–90+ and takes two to three.
That is the decision in one paragraph. The rest of this article is the reasoning behind it — what a fill physically is, why the window is two to three weeks, and why the "just fill it one more time" instinct eventually costs more than the reset it was avoiding.
What a fill actually is
Nails grow about 3mm a month, and product does not grow with them. Two or three weeks after your appointment, a naked gap has opened at the cuticle and the whole architecture of the set has migrated toward the tip. A fill is not "touching up the polish." The artist files down the surface of the existing product, fills the grow-out gap with fresh gel or acrylic, rebuilds the apex — the structural high point that has now slid too far forward — rebalances the shape, and re-does colour. It is real structural work on the original foundation: renovation, not repainting. Which is exactly why it only works when the foundation is sound.
The two-to-three-week window
Fills have a schedule, and it is not arbitrary. Before two weeks there is rarely enough grow-out to justify the appointment. Past four, two things go wrong at once: the gap is now wide enough that the set looks unmistakably overdue, and the apex has travelled so far toward the free edge that the nail is bending in ways it was never engineered for — which is when snapped corners and stress cracks happen. The sweet spot is week two-and-a-half to week three, while everything still looks respectable. The honest lifespan of a gel set and this fill window are the same clock read from two sides: clients feel it as "my nails are starting to look grown out," artists track it as the rebooking moment — it is precisely the window Nailly's refill reminders are built around.
When a fill is the right call
All four should be true:
The grow-out is modest. Under roughly 30% of the nail. At that point the existing product is still the majority of the structure and worth preserving.
Nothing is lifting. Run a fingertip along each edge. If any nail has an air pocket — often visible as a slightly cloudy or shadowed patch — that nail cannot simply be filled over. Sealing moisture under product is how green (pseudomonas) spots happen. One lifting nail can be removed and replaced during a fill; five lifting nails means the set is done.
Same system, similar look. Fills work within the material already on your nails. New colour over the same structure: easy. Gel over acrylic, or a rebuild in a different product: no.
Same-ish shape and length. Going slightly shorter or refining the same shape is fine. Coffin to almond means filing away the very corners that define the old shape — at which point you are paying fill prices for new-set labour, badly.
When you need a fresh set
Lifting on multiple nails, a crack that crosses the nail, or any suspicion of moisture underneath: reset, no debate. Changing shape or length dramatically: reset. Switching systems — say, after reading up on which set type actually fits your month and deciding to move from acrylic to builder gel: reset, with full removal between. And then the quiet one: age. Even a well-maintained set accumulates history — each fill adds product, thins the original layers at the edges, buries small imperfections under new ones. Most artists cap a set at two or three fills before insisting on a fresh start, because by then the oldest product on the nail is nine weeks old and behaving like it.
The economics, honestly
The cycle most regulars settle into: new set ($75, say), fill at week three ($50), fill at week six ($50), reset at week nine. That is $175 for nine polished weeks — about $19 a week — versus $225 for a fresh set every three weeks. Fills are the efficient path, but only inside their window. The false economy is stretching them: a week-five fill on a bending, half-lifted set is slower to do, more likely to fail early, and often quoted higher than a standard fill precisely because it is remedial work. Skipping the fill entirely and letting the set ride to week six then paying for removal, repair of the peeled layers underneath, and a new set is the most expensive schedule of all. If you want the set to actually reach its fill date in good condition, the week-three maintenance protocol is the companion piece to this one.
How your artist decides — and how to make it easy
A good tech makes the fill-or-reset call in the first sixty seconds: edges checked for lift, a press on the free edge to test flex, a glance at how far the apex has drifted, a question about what look you want next. You can pre-empt the whole negotiation when you book. "Fill, same shape, new colour" and "I want to go from square to almond" are different appointments with different durations, and telling her which one you are booking is the difference between a relaxed slot and an overrun one. Artists who take bookings through Nailly see the next design attached to the request itself — same layered spec, new look — so the fill-versus-reset call is often made, correctly, before you ever sit down.
The one-line summary: fills preserve a sound structure; new sets replace a compromised one. Respect the three-week window, listen when your artist says "this one's done," and the math takes care of itself.