Sam,
You can pretty easily forecast the value from now till you empty setpoint and then display the result as a Capsule on the screen as well as a Time/Duration in a Scorecard/Table and have all the results update in real-time
For my example I am going to use some example data but this should look very similar for your use case
Step 1 - Create a Forecasted Value - This function may not work on exactly as you expect on historical data depending on your datasource
$signal.forecastLinear(1.5h,5d)
//Train in the last 1.5 hours of data
//Project 5 days into the future
Step 2 - Create a condition that captures the time between now() and when you fall below your "empty" threshold. This step will only work for online data as it is using the past() operator
$LowerLimit = 0%
$AboveLowerLimit = $fv > $LowerLimit
$AboveLowerLimit - past()
Step 3 - Create a Scorecard to Quantify the time between now and full and display it in a table. This uses the "Condition" mode in the Tables view and a Condition scorecard type
Shamus Cunningham's post in Real-time Storage Emptying Time/Date Calculation was marked as the answer