When is it time for a fill?
Look at the cuticle line. When there is a visible band of natural nail between the cuticle and where the product starts — roughly two to three millimetres, usually two to four weeks after the set went on — the balance point of the nail has moved forward and it is time. Left longer, the weight sits too far out and the free edge starts to lever the product away from the nail plate.
Three other signals worth acting on: a hollow tap when you tick a nail against a mug, which means air has got under the product; a lifted corner you can catch a fingernail under; and a set that has gone dull all over rather than in one spot, which is surface wear rather than a structural problem.
People who type all day tend to be at the two-week end. People whose hands are mostly dry and still get closer to four.