The apex moves
Nails grow about three millimetres a month. An apex built a third of the way up the nail is, three weeks later, sitting three quarters of the way up and no longer over the point where the nail bends.
At two weeks it has moved a little and can be extended: the base is cleaned, new product laid on the new growth, and the high point walked back to where it belongs. Ninety minutes.
At three weeks the geometry is wrong enough that extending it produces a lumpy overlay with two high points, so the whole thing is thinned back and rebuilt. That is why the price is higher and the appointment longer despite looking like less work from the outside.
What a good removal looks like
The top layer is filed off to break the seal, then acetone under a wrap for ten to fifteen minutes, and then the softened product is pushed off with a wooden stick. It should slide. If it does not slide, it goes back under the wrap for another five minutes.
What should never happen: prying with metal, filing down to the nail plate, or an e-file on the natural nail at any point. Every one of those removes nail rather than product, and it is what people are actually describing when they say gel ruined their nails.
Taking a break
Not necessary if the sets have been applied and removed properly, and we will say so rather than sell you one. If your nails are genuinely thin, a light soft gel overlay protects them while they grow out better than bare nails do: bare damaged nails catch and tear, which sets you back further.
If you do want a break, take three months rather than three weeks. A nail plate replaces itself in about five months, so anything shorter than that is not enough time to see a difference.