There are system variables in Adobe Captivate that allow you to display certain pieces of information. For example, there is a system variable cpInfoElapsedTimeMS that returns the time in milliseconds since the Captivate project began playing.

You can add a shape or text caption to your slide and edit the text in that object. Add the contents of that variable using the [x] icon in the Properties Inspector like seen in the attachment.

Of course, the problem is that displaying the number of milliseconds isn’t all that useful to most learners. You would want to write your own advanced action to convert that to minutes and seconds or at the very least minutes. You could create your own user variable and then on each slide entry, you could calculate the number of minutes that the user has been in your course. This is a bit more complicated but can be done.

Here is a full list of system variables that are accessible to users to give you some inspiration for your own advanced actions.

https://helpx.adobe.com/captivate/using/captivate-variables-list.html