Ion motors are very cool stuff. So cool that Nasa decided to invest research effort in this subject since the early 1960's when the physicist Harold R. Kaufman achieved the first design. In spite of the small ammount of thrust (when compared to conventional rockets), ion motors are capable of a very large exhaust speed, being equivalent to ten times the exhaust speed of a high quality rocket (the later can only achieve between 3-4 km/s). This means that ion motors are capable of continuously accellerating an object up to the exhaust speed (considering the space environment, where drag is practically absent). The time for exhaust speed to be reached depend of the mass being moved.