It can take about one month from the receipt of a complete RDA filing.
Once you have a hearing and there are no problems with your project (which can send it into extra hearings), it takes about two weeks from the close of the hearing until you get your permit, which is called a Request for Determination. The permit is good for three years.
Planning for the Future
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.