How long will a CR2032 battery take to exhaust powering two 5mm blue LEDs
2 blue leds, so typically 3.2~3.6V forward voltage drop at 20mA forward current, will run at a much lower current when the voltage across them are less than that typical forward voltage.
They are likely to be running at 5mA or less, when the CR2032 is fully charged at 3.2V. From a random blue led data sheet http://descargas.cetronic.es/WW05A3SBQ4-N.pdf:

And a current vs capacity graph of a CR2032 https://www.ept.ca/features/designing-sensor-based-iot-devices-coin-cell-batteries/:

As the available voltage drops, the leds use less current, so they last longer too. It's like a see-saw where the led current and the battery voltage kinda level themselves out nicely. So between you not calculating the correct current drawn, and the curve of both the led and battery, the actual lifetime of the setup will be much longer than a general formula of (Battery Capacity in mAh / current draw) would tell you.
Leds do not have to run at their normal max current. That's just when they are their brightest. But they are still bright at lower voltages/currents.