Welcome to Android Central! The percentages aren't that helpful unless we know how long your battery is lasting, how much screen-on time you're getting per charge, how much time without cell signal you have, etc. If you're doing nothing with your phone, then the OS will likely be a large percentage of use, because it's always going to be doing something in the background, and if nothing else is going on, OS will use a higher percentage of the battery. But the absolute use may not be much.
To give you an idea, 3 hours of total screen-on time is pretty standard for the Nexus 5. Expect battery life to be worse if you often have poor cell signal, since it takes battery to search for weak signal.