What iPhone, what power adapter?
I can tell you my 6s Plus and 6 Plus before it would charge at a 1% increase per minute from 0% to 90% and then gradually drop to trickle charge. This was using the 2.1 amp iPad power adapter. Total time to 100% from zero took 1 hour and 55 minutes.
Any charge within the 0-90 percent range took one percent per minute so 15 minutes charge would increase battery level by 15%. 60% at start 75% after fifteen minutes. But you have to use the 2.1 amp power adapter. Not the little cube power adapter the phone came with.