During digging the percentage and remaining time is displayed to see the progress. It would be useful to also show the remaining time during upgrading the hospital.
It's not necessary to change the UI. It would be enough to show the remaining time in a popup text when the user clicks/taps on the upgrade progressbar just like when we see a popup touching the army/money label on the top-right corner. And maybe this progressbar-popup would be useful in smelters/crafters as well.