gmail

Let's say you want to AUTOMATICALLY delete all "Promotional" emails from your Gmail after 90 days.

Step 1:

Go to Settings -> Filters -> Create a new filter

Step 2:

Type the following into "Has the words" field:

category:promotions older_than:90d

Step 3:

Click "Create filter with this search"

Step 4:

Select "Delete it" and finally click "Create filter" button... 

And... we're done!

The key here is older_than:90d, you can just as easily create whatever time-based filter you want with that.