I'm dealing with something similar. There are certain tasks that typically take 1-4 hours. If you get lucky and it's simple one might take 15 minutes. If you get unlucky and it's complex it might take 2 days or more. Furthermore, there's another type of task that takes 1-3+ months. At a high level these two types of tasks look very similar. So people who typically request the first type and then need the second one once get upset that it's taking so long.

Plus just so much of it depends on what the current workload is. If I'm booked solid for the next 2-3 days then your 20 minute task is going to happen whenever I have the time even if when I'm less booked I'd have it turned around in a few hours.