My understanding is you can expect about 18 months from the C1. But the visual signs of the product being there (like the strong water beading) fade long before that.
GTechniq will tell you that the coating is a permanent bond with the surface, and therefore it is permanent and that the lack of water beading is caused by contamination and not washing your car properly.
I say BS. If the water beading disappears, then whatever was causing the water beading has disappeared too. Just because someone pleads not guilty, doesn't mean they aren't guilty.
Opti-Coat on the other hand just keep going and going and going. Is well priced, and requires such a small amount of product for application compared to C1 for example. It is clearly the leader in this category.