Your formula doesn't seem to take into account two important factors: layer height and average speed.
Do the math differently:
voumetric flow rate [mm^3/s] = layer height [mm] * line width [mm] * speed [mm/s]
For example, 0.2 * 0.45 * 70 = 6.3 mm^3/s
which is realistic, the extruder on the Ender 3 cannot go much faster than that reliably.
PLA weighs 1.24 g/cm^3 = 0.00124 g/mm^3
To extrude 1 kg you need 1000/(0.00124 * 6.3) = 35 hours (70 for 2 kg).
However the printer does not reach 70 mm except on straight long moves, the average speed may be lower or much lower depending on what you print.
If you print technical parts with straight edges, maybe you can multiply by 1.5 (50 hours/kg), but if you print models and small statues or similar you may need to double it (70 hours/kg).
I use Klipper as firmware and I have a macro which keeps track of printing time and filament length used.