From my limited experience, I have basically get a 6 month life on the G5 transmitter from when it shows up at my door. I posted some information on this from a Canadian perspective in another thread which is linked at the bottom of this post.
I made an attempt to always keep one G5 transmitter spare and stretched my first transmitter to 6 months using xdrip+ because my insurance only covers 1 transmitter every three months.
Dexcom Canada says (which is different from Dexcom US) - “Your transmitter is covered under a limited warranty for three months, starting from the shipment date.”