Hey Rose
What you need is a formula to track the time that takes you to complete a task from creation until is completed (correct me if I got it wrong).
The formula you need is: Completed on - Created on, both are native fields in every projects in Asana, but doing this as a simple formula will give you a result in a format that you may not like (in weeks and days), so I highly encourage you yo read this thread from Richard_Sather with some tips on improving your formulas in Asana: 🧪 12 Formulas to get you started with the new Advanced Editor
Basically you need this formula in your project:
( [[$COMPLETED_ON]] - [[$CREATED_ON]] ) / 1440 + 1
That will give you the time that take a request from your form (which are tasks). In my case I use days, read the thread I mention above where you can see his different tips depending how do you want to track the time that takes the task to be completed:
I hope this is what you are looking for to track in Asana.