Hi @nixonmark3, thanks for your query. Assuming that you are using either the Tobii Core SDK or new Interaction Library, then the simplest way to track how long a region receives gaze data would be to start a counter loop within the function that run when a gaze aware region receives focus.
DotNet has a stopwatch class within System.Diagnostics that should be adequate for the task.
If you take a look at our new Interaction Library which you can download the Beta @ https://developer.tobii.com/consumer-eye-trackers-interaction-library-getting-started/
Within the C# Sample, you have the function intlib.GazeFocusEvent += e => to receive gaze event. You could place your timer within here.
Similarly, this could be applied within the Core SDK using the WPF framework for which you have a number of samples to investigate @ https://github.com/Tobii/CoreSDK/tree/master/samples
Hopefully this points you in the right direction, please try it out and let us know how you get on.