The Appalachian Mountain Club's White Mountain Guide uses a very approximate formula to estimate the time a given hike will take. They calculate 30 minutes for every mile, plus 30 minutes for every 1,000 feet of ascent. Descent is assumed to take as long as walking on a flat trail.
BOOK TIME CALCULATOR
A more sophisticated (and hence more complex to use) calculator can be found on the NH Hikes and Walks site.