It depends on the judge. At my ceremony, they had everyone line up twice (that's government efficiency for you) to hand in your Green Card, then to pick up your Nat Cert. In between, the judge came in, gave a little speech, and he read the whole oath, at the end we all said "I do". Other judges may have you read it en masse. It took around 90 minutes total, most of it taken by the lining up twice.