On your website, you are asking your visitors to make a long operation, for example filling a form, and you want to know what is the average time took to accomplish it. We will describe actions from a general point of view, but by following those pieces of advice, you should be able to create an event using advanced data based on user action. Since this event is user action-dependent, the trigger should be the "page interaction".
To use the page interaction trigger, please refer to the relevant article, you should select an element and an action on your website that will clearly show the end of the operation you want to track. And since sometimes, selecting the action on an element is not enough, we also recommend you to limit this event to a portion of your website as described above.
By using the "Time on page" value and connecting it to the Google Analytics app, you will be able to directly use it in the report. Open it, select track time and please give a look to the third row, "Time (num)": a dynamic value has been selected.
Once done, you can save your event and begin to test it.