Doing nothing else. Just a static Google browser screen.
At 70% screen brightness
15 minutes equals a ~7% drop = ~28%/hr or less than 4hrs of screen time available.
At 30% screen brightness
15 minutes equals a ~5% drop = ~20%/hr or approx. 5hrs of screen time available.
At 10% screen brightness
15 minutes equals a ~4% drop = ~16%/hr or just over 6hrs of screen time available
Obviously all these numbers are relative to my other usage and are really just estimates as battery meters are far from precise (your phone is really guessing). Plus, they don't do decimals so I don't know if the drop was from 31.9% to 25.0% or 31.0% to 25.9% - hence my approximations.
But its still good to know and helps answer the questions "What's killing my battery?" and "Is this normal?"