I got the Perdix II in February and used it for a 3 month trip. The only thing I regret is not getting it sooner. Vibration alarms are great since I can't hear the high beeps of my previous computers and it is nice to get a subtle heads-up when needed. It feels rock solid and readability, information content, data download all work just great. Dive computers with color LED screens use a lot more battery power than the passive LCD screens used in the past. So if older Perdix computers refered to in other replies didn't have the color LED screen than battery life comparisons are probably not informative.
I use two rechargeable nitecore 3.7V Li-Ion batteries. I already use bigger nitecore 3.7V Li-Ion batteries for my backscatter videolight and can use the same charger for the dive computer battery. I swap batteries after the charge symbol turns yellow. The computer can actually tell you the battery voltage and I found it switches to yellow at 3.7V, so there is a lot of battery power left (a full battery starts at 4.15V and I think it can drop to 3V or less). However, a battery swap is so easy that I don't wait for it to turn red. I ended up swapping batteries after about 4 days with 5-6 hours of bottom time per day. If you wait till the battery symbol turns red you can probably count on one swap per week.
I do recommend to have a spare because the degassing computations stop when you take the battery out and then restart when you put a new battery in. If it takes you 2 hours to charge the battery you have missed 2 hours of degassing. It is a 'conservative mistake' but if you want to charge overnight you definately want to have a spare and not miss 12 hours of degassing calculations.
A final point is that with my old LCD computers battery life depended mostly on the number of dive days, not the number of dives, because the screen uses little power and deco calculations continue for 24 hours whether you make 1 or 3 dives. For LED screen computers battery life depends more on actual bottom time because that is when the screen is on.
