Average iphone SE usage time?

See these Apple tips for maximizing battery life:

https://www.apple.com/batteries/maximizing-performance/

If you purchased the phone from Apple, you also have 14 days from day of purchase to return it.

The usage time is going to vary depending on what you do on the phone, what features you have enabled, your brightness setting, the ambient temperature, your signal strength. Unless you exactly match the "average user", that information won't help you very much.

Also, it's important to remember it's not 10-12 hours usage AND 21 hours standby. It's up to 10-12 hours usage OR up to 21 hours standby.

The numbers manufacturers give is usually a best case scenario.

As mentioned, the specs say "up to" 10-12 hours. Many users will get less, some may get more. it depends entirely on you and your specific use of your device. Also, that number will inherently decline as the device ages and the battery degrades naturally with time and use.

Define "normal" use of facebook or whatsapp? Are you sitting there with the screen active for hours at a time? Do you engange in lengthy chat sessions with people? Are you streaming live content from FaceBook? Do you have the screen brightness turned all the way up? How many notifications are you receiving? Do you use push email? Do you use location based alerts or notifications? Are you sharing your location, or using anything else that uses location services a lot? Is your wifi signal strong, or weak?

There are millions of iPhone users, and there are millions of apps. There are people who spend hours a day on facebook, and some who never use it (some of us have never used Facebook, or whatsapp, and never will)? When you say "normal use" that is a meaningless phrase - it tells nobody anything at all about your actual usage of your device, since nobody can possible define "normal use" of a smart phone. What's normal for you will not be remotely normal for someone else.