Every route on your map shows something like Est. 3 min · 180 m. Two numbers you control produce it, and both live in the Walk time & distance settings.
The calculation
It is two simple steps. First distance = path length × scale, where path length is measured from the drawn route line. Then time = distance ÷ walking speed. So the metres depend on your scale, and the minutes depend on the metres and your speed.
The fields that feed it
Scale. With Auto-calculate scale from GPS ticked and all four GPS corners set on every level that has an image, distances are computed per level from those coordinates. Otherwise the Scale, metres per map unit field is used (a rough 0.35 until you set it).
Walking speed. The Walking speed (m/s) field sets the normal pace, default 1.3 m/s.
Accessible speed. When a visitor is in step-free mode, the Accessible speed (m/s) field is used instead of the walking speed.
Make the estimate trustworthy
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Get the scale right
Prefer GPS corners on every level for accurate per-level distances, or set a sound manual scale.
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Match speed to your visitors
Slower audiences need a lower walking speed so times are not optimistic.
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Sanity-check a known route
Compare the metres shown against a distance you can estimate. If it is consistently double or half, the scale is out by that factor.
For the deeper walk-through of scale and GPS, see Get accurate walk times.