T... and gravity (maybe a constant); assuming shot perfectly parallel to a surface.
It isn't TRULY a constant... depends on the distance from the mass center of the Earth, but for calculation purposes acceleration due to gravity is deemed a constant of 32.2 ft/s^2.
If you are doing a calc that requires more precision, there is a formula to convert based on elevation above sea level.
The time it would take for a bullet to fall to the ground can easily be determined by t=mgh.