^^^
THIS
Another thing to consider - are you going to leave it stock or run a custom rom? Custom kernels can add a few hours to your battery. With that being said, the extended battery gave me an extra 3 hours of use. I get anywhere between 12-18 hours depending on my screen time on and how hard I hammer it during the day. Just to be safe, if I have access to a charger, I always charge it so I don't have to think about it.
EDIT:
The two things that kill the battery on the Nexus
- Screen - if you can lower the brightness this will help tremendously
- LTE - if you can connect to wifi - do it, it will help a lot as well