I stopped using them.
The length of time they stay on varies from cat to cat. They come off only when the cat sheds the outer layer of nail, and this is what varies. What I found on the majority of the cats I tested is that they lose 1 cap at a time forcing me to constantly be replacing caps (1 at a time). Only 1 cat actually kept them on for a good amount of time, 5 weeks.
They are not difficult to apply, but the cats tire of the routine quite quickly (after about 3 replacements most of the cats were growling and biting to make me stop).
My biggest reason for discontinuing use was I found them to be the cause of freak accidents, ones that could have resulted in injury if I were not there to monitor. Since the caps are super-glued into place, they do not fall off and they grow out as the nail grows out. This causes a rim to form between the cap and the cat's cuticle. I twice had to rescue a caught claw that was hooked on a string (from the scratching post carpets) that was too strong to break and now hooked tightly on the rim of the nail cap. I shudder to think how hard the cat would have pulled to release itself if I were not there to rescue.