I have battery saver set to 15% on my Nexus 4, and just recently set it to do the same on my fiancees Nexus 5. She thought you could only turn it on manually, so showing that it'll auto activate just blew her mind.
It is quite good at making your phone last while on that little bit of charge. However, I noticed that anything that uses a good bit of resources does still drain the phone at a fast rate. In my experience, Chrome and Flipboard can knock off a percent every few minutes, but an occasional text doesn't harm it too much. Overall, the phones last a heck of a lot longer than just leaving them be without battery saver. Have yet to activate it on either of our Nexus 7s though. I'm not even sure that we looked to see if hers can be updated yet (2012).
I haven't gotten quite 2 days out of my phone yet, but I'd say about 30-36 hours is a good estimate if I don't use it too much beyond the occasional text message. This is also with a constant connection to my Moto 360, and connection to my car's Automatic sensor. I charge it nightly anyways.