I try not to put the answer in terms of time ("about another 20 minutes brings you to the creek.") They're not hiking the same speed I am, so I try to answer in distance. If I know, pretty closely, I'll tell them ("Well, I passed that trail junction probably half a mile back, and it says it's 4 miles from there to the falls.") The half mile may really only be a quarter mile; I estimate high if I'm uncertain. If I don't know, or there isn't a trail marker to go off of, I'll pull out a map, say "we're about here, and you want to go there. It looks like..." and we'll work out the mileage together.
The sad part is, they sometimes don't have their own map. I've given up a Forest Service or National Park map a number of times, and even parted with one or two topos. I don't know if it helped; I purposely avoid the "Lost hiker found..." articles for a couple of weeks after that.
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