How long do Vivera retainers last?

I finished my Invisalign treatment at the end of 2015, and decided to go with Vivera retainers. I wore my last Invisalign tray for a month while I was waiting for my retainers to come in. I received 4 Vivera retainers and all I was told was one of the retainers was a bit thicker to correct tiny gaps and the other 3 were identical and to keep around as backups. Was this not correct information about these retainers, and will I have to replace my all 4 of my retainers in a year?!