How to calculate how long a 9V battery will last based on schematic?
It's not clear whether your running the LEDs at different brightness is intentional or accidental. The small difference in brightness is not large, so I'm guessing it's accidental. Two configurations that run the LEDs at the same brightness would be ...



simulate this circuit &ndash; Schematic created using CircuitLab
The first will operate down to the lowest battery voltage. However, the second draws half the current for the same light output, so may last longer anyway, even though the battery voltage will be higher when the LEDs reach an unusable brightness.
The nominal capacity of an alkaline PP3/6LR61 is 0.5 Ah, though it's not always clear what end voltage or what rate of discharge manufacturers are using when they test this.
If the second configuration takes 10 mA, a ballpark lifetime would be 500 mAh / 10 mA = 50 hours. That's maybe within a factor of 2 or 3 either way, given the uncertainties of how the 500 mAh is specified, whether your particular battery is a quality one or a knock-off, what your end point for subjective brightness is, and the fact that I haven't even attempted to allow for the reducing current as the battery runs down.