How long would it take for a 12U cubesat on a circular polar orbit with altitude 600 km to fall down to 500 km?
Please, add references/calculations to your answer.
There is no one answer to this question. How long a 12u cubesat will remain in orbit depends on a number of factors, including solar activity, orientation, and mass. Atmospheric density at 600 km varies by well over an order of magnitude over the course of the eleven year solar cycle. Flying with the long axis of the satellite along or across track changes drag by 50%. Since drag acceleration is inversely proportional to mass, a very light 12u cubesat will suffer greater drag acceleration than will a very heavy one.
That said, a 600 to 700 km orbit is on the cusp of violating the 25 year lifespan used by NASA and ESA. If your 12u cubesat flies such that the small face faces the wind and the cubesat mass is over 17 kg, the odds are quite high the spacecraft will remain on orbit for over 25 years.