I've been having my three nails done for almost two years. I've had the acrylic pulled off once or twice when a major repair is required ('cause I've smashed the nail and acrylic) - and each time I've checked the thickness of the underlying finger nail. It has always been normal. I have never had the acrylic covering soften the underlying nail. I don't know if the technique they use is different, but I know my real nails are quite healthy.

I do go sometimes as much as four weeks. But usually I go 3. They grow pretty fast and after three weeks I've got weapons on my hand, if I don't trim 'em or have the nail ladies do it. It's a good system.