That's a hard question to answer, because it's a percentage, and it may differ widely depending on how heavily you use your phone for various activities. For example, on my N5, OS is currently at 13% and System is at 10%. But if I suddenly started playing a lot more games or listening to a lot of music, then the percentage of battery use for those games or music apps would go up, thus making the OS and System percentage go down, because it's all relative.