D tpa, DJI recommends to keep battery at 60-70% charge level if you don't use it.
As for critical levels. On my Phantom 2 Vision I never flew once batt hits 30%, 30% is when I start to land no matter what. I had very sad expeirence once, batt went 30% I tried to return home, but seems like voltage wasn't enough and it started to descend even with throttle stick full up. I managed to land it like its ****** F16, but was lucky to have clear surface by.
My creed - never fly under 30%. Always keep an eye on batt level and remember there's a point of no return. I usually land my phantoms at about 35-40%
Better buy couple of extra batts when risk with extra seconds flying and loosing bird
Safety first, folks!