It sounds like you are going to Settings > Battery and you get the message that the info will be available in a little while in the 'Time since last full charge' section at the bottom. When you check this section when the phone is still at or near 100 percent charge this message is expected. If however you never see this update correctly even after a prolonged period of use, let's start by quitting the Settings app:
How to force an app to close on your iPhone, iPad, or iPod touch
Then restart the phone and use it for a while to see if this updates:
Restart your iPhone, iPad, or iPod touch
Have a good one!