20 hours has past on standby, 7 hours of usage and the battery symbol still is full. That is good news.
A bit more confusing news is that "Usage - 7 Hours, 5 Minutes" in settings screen. Before I went to bed the phone had no more than 49 minutes of use. I didn't use it till this morning so I'm a bit confused about that indicator. Nevertheless, full battery is a good sign, and I'll keep you posted on when the battery finally dies.
Few days have passed, and iphone's battery is great. So finally, I mean, really finally I can say that I managed to fix fast battery drain the iPhone had because of wifi chip.