PSI is PSI so doesn't matter if it's a spare or on the truck, the time will be the same.
While it may have only taken 90 seconds to air up from 40-50, it will take longer as the pressure increases. So far a valid test, try the 40-80 psi test. Plus, it is more common to air down to 30 psi or lower depending on how soft the sand is.
The problem with the cheaper units is they will fail when you need it most. Not so bad if you only air down at popular spots when there are others around you, but terrible if you travel out in the boonies where other help isn't close at hand. That is why I spent the money on a better unit, so when I have it when needed with little chance of it failing.
