Actually I'm having the same problem: ROG G750JZ purchased 7 month ago, before 2 weeks ago the battery last easly 6 hours of web browsing...then the battery life drop to 3h!
It happened too all of a sudden to be the normal battery deterioration, and I'm still trying to figure out what specifically caused this problem.
Here is the details of my situation before and now:
Laptop configuration:
Asus ROG G750JZ - Intel Core i7-4700HQ - Nvidia GTX880M (4GB) - 16GB - Windows 8.1
SSD (slot 0): Samsung EVO840 256GB (added next, see below)
HDD (slot 1): WDC WD7500bpm ~700GB
Note: all the time measures are referred to a usage of web browsing (if wifi turned on) , document reading and/or word processing on intel GPU ( thanks to optimus ), performed from 100% to battery complete discharging
-From July 2014 to December 2014
Laptop in factory condition, only HDD in slot 0
Battery life: wifi ON ~6h ; wifi OFF ~7h
-From (ending of) December 2014 to first half of February 2015
Samsung SSD mounted on slot 0; fresh install of windows 8 through "asus backtracker" on SSD; HDD moved to slot 1 as data only storage; SSD optimized throught "samsung magician" software.
Battery life: unchanged, wifi ON ~6h ; wifi OFF ~7h
-From half of February 2015 to (now) end of February 2015
Laptop mounts both SSD (Win8.1 here) and HDD
Battery life: ~50% of precedent performance; wifi ON ~3h ; wifi OFF not-checked-yet
I started having various problem: windows indexing service running "heavily" also in idle (~20% of CPU always dedicated to this process, CPU fan run consequently ), strange issues about system permission (like I couldn't turn on plane mode option or switch off bluetooth) probably because of a wrong setting of my home network, also optimus seamed to not work properly.
Formatted through "asus backtraker" performing a new clean installation of the system (Windows on SSD) -> solved permissions and (supposed) optimus problems; done every windows updates; windows indexing still running wild; Battery life still low; windows indexing solved disabling it at all; Battery life still low;
Formatted (again) through "asus backtraker" performing a new clean installation of the system (Windows on SSD) -> windows indexing immediately stopped, no CPU over-usage or unexpected fan run; no windows update made; Battery life still low;
What tests I'm going to do now:
-using only the HDD removing the SSD
-using a different battery
-test my brother ROG G750JZ (same laptop configuration as mine except for the SSD) that has never be formatted or upgraded and has been purchased on October 2014
-using linux both on HDD only and SSD+HDD
For now I couldn't identify the cause of this problem, that to me seams all but a normal battery consumption.
Any help or suggestion?