You may not delete data from an extract.
If you're willing to use a full extract (vs. incremental) you can apply a relative date filter so that it only grabs the last X weeks of data, therefore you'll get a rolling window.
Some people will combine two refresh schedules with a relative context filter on the viz or data source itself to enforce last x weeks. Here's how it works:
- Add a "last 5 weeks" filter to the data source itself - now you'll only see last 5 weeks, even if there are 6 or 7 weeks of data in the extract
- Use a nightly incremental extract to add more data. By the end of the week you'll have 6 weeks of data
- Use a weekly FULL refresh to start again - 5 weeks.
Hope this helps!