Er, if the people doing the looking are idiots, then yes it can take that long. You are ignoring that and assuming the people examining the pictures had their brains switched on. They didn't. When someone who was sensible and clever looked at the pictures in later years, they spotted flaws and confronted the old crones about it, who fessed up.
Actually, if I recall it was James Randi who first proved them to be fake, and did so with computer technology which did not exist until then. Lots of experts thought they were fake, simply because they didn't believe fairies could exist, but they couldn't conclusively prove it.
The girls father thought they were fake at the time they were taken. Many people since then have accused them of being fake. If I see a magic trick happen and a rabbit disappears, it doesn't mean I believe the rabbit has really disappeared. I know it hasn't but I cannot explain how the effect was produced. It's the same with the fairies. Most people examining the photos seemed to be looking for signs of photographic manipulation, which was not happening - the photos are in fact undoctored, which is why this sort of tampering was not found. However there are clear signs which should have been apparent to everyone who saw the pictures, as I will highlight here:
This is the most well known photograph and depicts fairies in a brisk dance, frozen pin sharp as if by a fast shutter speed. However, the blurred water in the background indicates a shutter speed of around 1/4 to 1/2 a second was probably used, which would have reduced the sharpness on the fairies considerably. Notice also some blur, probably from a slight breeze, on the girl's hair at the top.
The same problem is visible very obviously in this photograph. The girl is blurred from movement (it must be movement and not poor focus because features nearer and farther from her are sharp) whereas the fairy is again unblurred and frozen, even though it appears to be flying, and its wings would be flapping.
This effect SHOULD have been obvious to anyone who understood the slightest thing about photography. The fact that it wasn't noticed for years does not mean the pictures are convincing - it means people see what they want to see and don't have their brains switched on.