The process usually takes 1 to 2 months. If there are several projects on the agenda, or if there are complications or discrepancies with an application, it can take longer.
Once you have a hearing and if there are no concerns with your project (which can send it into extra hearings), it'll be about 2 weeks from the close of the hearing until you get your permit which is called an Order of Conditions.
Note: Even once you have a permit, you cannot start work on you project immediately. You must record your Order of Conditions at the Registry of Deeds and you must wait out a 10 day appeal period.
The permit is valid for 3 years from the date of issuance.
Get a Permit in Advance
If you know you're going to do a project in the near future, you can get your permit a year or more in advance and save yourself some stress.