###This script will trash all gmail emails with the labels specified, after the number of days defined (in the listOfDays array).###
Check out the tutorial and script.
##T U T O R I A L##
Step 1. Create gmail filter(s) to apply label(s) to certain emails in the format "NAMEOFLABEL[NUMBEROFDAYS before trashing]" (e.g. "delete3").
Step 2. Add gmail-time-based-trash-filter.js to your Google Drive and open script, or go here.
Step 3. Update the values of the following fields (and save the script after updating):
var labelTemplate = "NAMEOFLABEL";
//this field can have any amount of comma separated values
var listOfDays = [NUMBEROFDAYS, NUMBEROFDAYS];
Step 4. Go to Run -> Initialize and authorize the script.
Step 5. Go to Run -> Install to install the script.
You can now exit (Google script editor) at any time, and any email messages in Gmail folder(s) will automatically get purged after 'n' days (in listOfDays). The script will run by itself every day.
Also, you may go to Run -> Uninstall to stop the purging script anytime.
For information about the original script go here.