In the comics
Marvel's site states:
Quicksilver possessed superhuman speed, and could travel on foot at speeds exceeding the speed of sound for hundreds of miles before tiring; he could "fly" for short distances by flapping his arms or "vibrating" his legs; he could also use his speed to create cyclones, dodge machine gun and laser fire, and run up walls and across water.
However, his powers are amplified later on.
Following the loss of his mutant nature, Quicksilver exposed himself to the Terrigen Mists, resulting in him now possessing the power to vibrate his atoms so quickly he travels forward in time. His molecular speed that he generates displaces him out of the mainstream time/space so that he is able to propel himself into the future. He can leap from thirty seconds to up to twelve days, and remain for several minutes to several hours before being recalled to his present time once his body tires, or he can return at will before his time is up. As he returns from his trip, he returns the exact moment he left so as to appear that he has been gone for half a nano-second. He is able to bring inorganic objects from the future back to his correct time, although it has yet been shown what would happen if he attempted to bring organic objects with him.
In the X-Men movies by Fox
The movie's official website simply lists his maximum speed as "faster than the speed of sound".
In the Marvel Cinematic Universe
Unfortunately, no concrete information regarding Quicksilver's maximum speed has been made public as of yet. During the course of the movie, we see Quicksilver outrunning Mjolnir and arrows, as well as altering the trajectory of arrow in mid-flight. This is not necessarily indicative of the limits of his abilities. Even the junior novelization only describes his speed as "faster than a bullet train" and "faster than a jet".