I measured my office too, with an Adafruit SCD-30 sensor, it also got to 1500ppm faster than I expected. And it took a long time (12+ minutes with fully open windows) to get it down to an acceptable level again. Certainly compelled me to do that more often.

Surprisingly, i couldn't find any calculator or theoretical approach for estimating this (given room of a certain size, how long does gas need to equilibriate with outside atmospheric composition to within some tolerance, through a hole of certain size)