Unless the fan stalls due to Hall Effect position error in start position but starts with slight nudge, the fan current always decreases when V+ reduces to 3/4 voltage since less work is done and internal fan drivers are fairly efficient.
Since 9V Alkaline 6LR61 cell has 5Wh capacity at 20h and 3~4Wh in 5h
- and 12V 0.15A fan draws 1.8W with a fresh cell, power consumption will be less than 1.8 but more than 50%. I estimate 60% or 1.1W (from past fan DVT design validation tests.).
Therefore nominal run time will be expected to exceed 3~4Wh/1.1=~3~4 hours with declining RPM extending time at lower mA draw in exponential decay down to 4V then rapidly cutout and stop
RPM declines with Vbat and stall Voltage has hysteresis around 25% depending on bearing stiction from wear out.
If not using “top Brand quality” Alkaline, your results will be worse.