Hi,
There's a couple of approaches you could take here. . .
Natively Workfront will capture the assignment/reassignment of tasks as a journal entry (if you view the system activity on a task, details of assignments are logged here). Therefore, a journal entry report might be enough to get what you need. You can read more on journal entry reports here
An alternative approach (and my preferred, since the reporting will be more flexible) would be to create a calculated field that is attached to your tasks that will record a timestamp of when a task was first assigned.
In the below code example I have created a field called "Calculated Assignment Date" that will create a timestamp as soon as the task has a person assigned to it. Note that it will only capture the first assignment date, it will not update if the task is reassigned to another user
IF(ISBLANK({DE:Calculated Assignment Date}),IF(!ISBLANK({assignedToID}),$$NOW,""),{DE:Calculated Assignment Date})
Hope this helps!
Best Regards,
Rich.