It depends on your centrifuge. Measure the radius of your rotor in centimeters, then rpm would be square root of (g-number*8.95*10^4/radius (in cm)). Let say, radius of your rotor is 10 cm; then 300 x g is 1640 rpm.
The relationship between g and RPM depends on the radius of the rotor, which is the distance from the center of the rotor to the tip of the tube. The larger the radius, the higher the g force for a given RPM, and vice versa.