Freeing up space · Free Up Space wizard and button
Why does the progress view say "working out how long that will take"?
Short answer: Storage Studio can't estimate the time yet, so it says so rather than inventing a number. Once copying is steady you'll see a plain estimate like "12 minutes".
It means Storage Studio doesn't yet have enough information to give a sensible estimate, so it says so instead of showing a number it would only have to walk back.
Once an item is copying steadily, you get how far along it is and roughly how long is left, phrased the way a person would say it — "a few seconds", "12 minutes", "1h 20m". You don't need to watch it: moving files is a background job and carries on if you close the window. See the Show progress button.