A few years ago, I accidentally punched a few holes in my liner which my pool store patched with extra liner material and glue. It worked well but over time the patches started to lift at the edges. The liner was ten years old. Even with the patched holes, I thought the rate of water loss seemed high. I'd often lose an inch or more a week. Searching the various pool websites largely calmed my fears by attributing such loss to normal evaporation--I live in southern Mississippi. I was adding water every week.
This summer, I called the outfit that did the patchwork to do a re-patch but they were out of business so I chose the best looking ad from the Yellow Pages and called them. The owner came right out and with 10 years on the liner, suggested that replacing the patches was not a good idea--he recommended a new liner. He said it was probably leaking in many places and would not guarantee a repair.
So I authorized a new liner.
When they tore out the old liner, there were places where the old vinyl actually broke apart, it was so brittle. I could hear it snap as they folded up the pieces to haul them away. and the cement at the bottom of the pool was black with mildew from long-term dampness. The installer said I was losing a lot of water through minute, virtually invisible cracks and that water was percolating through the relatively porous concrete used to line the pool.
With a new liner, I now lose less than 1/4 inch per week. I have yet to add water as normal rain keeps the level at the overflow drain.
I'll not wait another ten years to replace the liner if I start seeing excessive water loss.