In terms of planning distances I figure 5:1 for elevation. That is, a meter up effectively adds 5 meters horizontally. This is true for up and down both.
In practice the up part takes longer for any but the most fit, but coming down is still slower than flat (you are picking your foot landing more carefully.) Both going up and going down you are taking shorter steps.
So a 20 km loop route with a 1 km total of up and down will take about the same time, and leave the same fatigue more or less as a 25 km flat route.
In more detail: 8% is a magic number in terms of gradients. Up to about an 8% grade (8 foot climb per 100 foot horizontal) you can walk normally. You have a heel strike. Your step shortens up hill and lengthens down hill. At 10% for most people, you change to a toe strike, and you are climbing, rather than walking. This is not nearly as efficient. You will find that the vertical change entirely determines your speed. E.g. when you are walking you do 3 km/hour. When your are climbing you do 1200 meters an hour up. Your numbers will vary.
But the 5:1 rule is good enough for me.