Time metric in Marketo: How can we accurately measure trends?

We are trying to accurately measure our audience trends, particularly how long it takes on average for a subscriber to advance in our engagement program based on email activity. However, it is almost impossible to gauge this without some sort of time metric in Smart Lists or Email Performance field.

For example: Person X was in [Segment] for ___ months/days.

Is there a way to add in a custom field to measure this? Or some other out-of-the-box method we can't brainstorm for now.

Thanks!