Whetstone Mountain is accessible from the Opal Creek trailhead which can be hiked from mid-May until mid-November, as well as an upper trailhead that isn’t accessible by car except during the summer and early fall months. The trail to Whetstone Mountain summit from Opal Creek trailhead is 11.25 mile round trip and the trail via the upper trailhead is 4.2 miles round trip.
If you are hiking on a weekend, remember to get to the trailhead early, as Opal Creek gets very busy and the trailhead fills up fast.