My iStick 20W goes about 8, maybe 10 hours of vaping at 1.8 ohms, 3.7v to 4v. That's about 8 watts, give or take. It's a 2200mah battery. Vaping frequency and load will determine battery life.
I'd recommend the iStick 50W, but experience with two of them resulted in the OLED screen going dark inside the warranty period. Maybe my vendor bought early on and their entire stock was cursed with the same build lot.
They replaced both and then let me choose another after the 2nd failure. I chose the Innokin Cool Fire 4. It's been rock solid so far and gives me vaping time from about 7 in the morning until about 8 or 9 at night on a charge at 8 or 10 watts.