Components necessary to automate hand crank
5 rotations per second is a lot. I wouldn't be surprised if the C nut and ACME rod that probably drives your desk were not actually rated for that speed.

To get a reasonable amount of torque at, say, 1 revolution per second, you'd use an electric gearmotor. Pololu.com has a number of good choices. For example, the 37D 5A gearmotor at 100:1 gearing will run at 100 rpm, giving you 200 oz.in of torque (which is a fair bit -- probably more than you're cranking on a 3 inch lever.)
http://www.pololu.com/product/1106

You also need a way of mechanically coupling the 6mm D-shaft output to the hex bolt, some way of fastening the motor so it doesn't counter-spin, and some way to drive the motor.
Driving the motor means a 5A/12V power supply, and an H-bridge that can take 10A/24V to safely withstand the worst-case reversing-direction transients. A Pololu simple motor controller, or an Orion Robotics RoboClaw, or a Dimension Engineering Sabretooth, would be a reasonable hobby-level choice.