The device itself is easily a 3-4 year device, it's built like a brick jailhouse, but I think the only way any Li-Po is going to last 3 years is if it's babied and discharged less than 10%, and even then, you're still going to lose 10% capacity a year just from oxidative breakdown. Some links you might find interesting:
How to Prolong Lithium-based Batteries - Battery University (Figure 1 and Tables 2 and 3)
Understanding RC LiPo Batteries subheading: THE 80% RC LIPO BATTERY RULE TO THE RESCUE!
http://www.rogershobbycenter.com/lipoguide/ subheading: LiPo Packs versus NiMH Batteries
I personally don't like built-in batt devices, especially those that employing 2 or more batts that take the Jaws of Life to get to. There's so much involved in keeping batts married I don't want to be bothered with it. And I don't trust device electronics to do it well either. Single removable batts I can toss every year for fresh ones are definitely my cup of tea. But that preference is my own paranoia, and, of course, to each his own.