Ever heard of Naismith's Rule? In a nutshell, add 30minutes to your time for every 1000ft of elevation gained. So, if you are traveling up a 3 mile trail at 3 miles per hour and it gains 2000 feet, it will take you roughly 2 hours (1 hrs for distance + 1 hr for height) to reach the top.
Naismith's rule helps with the planning of a walking or hiking expedition by calculating how long it will take to travel the intended route, including any extra time taken when walking uphill. This rule of thumb was devised by William W. Naismith, a Scottish mountaineer, in 1892. A modern version can be formulated as follows:
Here's the full Wiki article.