Go full robot: "Estimated timescale for the completion of this task?". I doubt they'll get annoyed at that, might just find it funny, or weird. no, definitely wierd.

Then you get a "Not long."

Sir I must insist on a standardized measurement scale.

The trick is to make up a deadline. "I need to get home to swap the Dilithium crystal in my lizard tank, do you think this will take longer than an hour?

I go with "Do you know roughly how long this will take? 5m, half hour, hour, three hours?"

The answer will be either higher or lower end answer so you can get a ballpark estimate.

"Just curious" seems to come across as passive aggressive.