26,666,666.6667 Miles per hour
actually impulse refers to any speed below warp so impulse varies from ship to ship
The Star Trek literature I've read concerning the speed of impulse usually suggests a speed of .25c (One quarter the speed of light) as the official designation for "Full Impulse", citing that at highter speeds relative time problems start to arise.
During combat, ships could be traveling just about any speed less than light speed. In the Last Unicorn Games version of the Star Trek RPG, Ships going .9c or .95c were not unusual.