this is about the shuttle, but its same for pretty much all rockets:
megan McKenna from Beecher, IL: After the shuttle launches, how fast is the vehicle going when the command is given to "throttle up," and how long (in seconds) does it take to reach 17,000 mph?
Leinbach: Another good question. The "go" at throttle up command is interesting because as we ascend through the atmosphere in the early stages of ascent, we go through a regime that's called the maximum dynamic pressure. The maximum dynamic pressure is a combination of the speed of the vehicle and the density of the atmosphere we are flying through. So, shortly after T zero, shortly after lift off, we throttle the main engines back down to around 64% rated power to keep that dynamic pressure on the vehicle to a minimum. If we didn't throttle down, the loads on the external tank and the solid rocket boosters and the orbiter would be too high because we'd be flying faster through this regime in the atmosphere called the maximum dynamic pressure. Once we get through that area, then it's safe to throttle back up and go for the maximum acceleration of the vehicle. That occurs when the vehicle is about 35,000 feet high. At that point in time, the vehicle is going 1,636 miles per hour when we are "go" for throttle up. Then the engines stay at the maximum power rated level all the way through ascent. We do throttle them back down slightly as we get really close to orbit to maintain no more than three G's on the astronauts and on the orbiter itself, but that is late in the ascent - maybe around eight minutes or so during the eight and half minute flight. So, the throttle up is to bring the main engines back up to speed, the full rated speed, once we get though that maximum dynamic pressure.
http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/sts115/launch/qa-leinbach.html
