An average person should be able to walk about 3 miles in 1 hour on flat ground.
Don't allow yourself to think that your trip back down will be easier or faster. It will be toward the end of your day, and many times walking downhill is more difficult on your body than walking uphill.
Depends on the load, but if your group is in shape then 50 lb load (backpack/ water/ equipment) = 4 miles per hour on grade, but your inclined 1500 feet so probably 2.5 mph on the way up = 4 hours total (plus meal and two breaks). The way down will be 4 mph at a leisurly pace. Plus stopping time of 10-15 minutes per hour to take it all in. I will tell you, since you asked how long it will take means that you don't do this very often. I would seriously consider limiting your hike to 4-6 miles tops. 10 mile hike is pretty serious the first time out if you don't do it frequently and coming down will be quicker as you will not have the same load as on the way up and it's down hill. I have hiked 100's of miles and uphill IS slower than downhill, but at least your starting the trip on the hard part while your fresh.
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