Most phones battery usually finishes after 4-6 hours of active use time (screen on time), so I believe using a phone for 5 hours a day is indeed 'heavy use'. 'Standby time' is not very relevant usually, it's the active usage time/Screen on time that matters.
Assuming 5 hours of active usage/Screen on time. Calculating battery timings.
For a person, who uses phone 2.5 hours a day, battery will last two days for him.
For a person, who uses phone for 5 hours a day after every two hours for one hour, battery will last 15 hours for him.
For a person, who continously uses the phone for 5 hours, battery will only last 5 hours for him.
So standby time varies from user to user.
