They are stating 120 days +/- from the date that your petition is posted to the public docket.
http://www.regulations.gov/#!home
It is important to note that what they are
not saying is that it can take 60-90 days to get the Docket Center to post it to the public docket for public & FAA viewing. This is true whether you send them an actual hardcopy petition or whether you post it to the Docket Center.
So.... 90 days + 120 days= 210 days...... about 7 months....
If you look at current Approved 333 exemptions and look at the date it was posted to the Docket Center & the actual date on the petition, you will see the current timeline....