we were told at college that there is no shelf life as such when it comes to nail enamel, so long as they have not gone all gloopy or dried out then you can make them last for ages if you take care of them.
CND enamels never go thick as there is something in them and for the life of me I can't remember what it is, but when you roll the bottle prior to use, the enamel goes all nice and fluid.
Someone else might beable to tell you more about CND enamels as I've not been using them that long, so I've never got to the end of a bottle!
My initial varnish range was Zoya (Supernails) purchased between 2-3 years ago, and although they may not all be used on a daily basis, most of them are around the 1/2 way point. These are still as good as the day I first bought them. My main stock now is CND, and once again these seem to do extremely well, although my Negligee varnish did go a bit gloopy in the last 1/4.
However, I'm a bit of a stickler when it comes to my polishes (Sleeping with the Enemy :shock: ) and all rims must be cleaned before the brush is replaced. This stops air getting in and drying the product. Another tip :idea: - don't keep plunging your brush back and forth into the bottle, as this will again trap air in ;-)
I think that what TC was trying to say is that CND enamels are thixotropic which means they thin with agitation. CND do not even sell thinners as the enamels (kept in the correct conditions) never go thick.
If by any chance something has caused CND enamel to be less than perfect, just add a few drops of Stickey Base Coat to the enamel and this will restore it.