I hear recommendations for FW dips from 30seconds to 10 mins all the time. I tend to do at least 5 mins, and closely observe the fish in a clear container. You can definitely tell when the fish has had 'enough.' They will become inactive, and usually lay down. It's a bit harder to tell with angels, because they typically turn broadside to anything, including the bottom of the container.
However, a FW dip is only a temporary relief for parasites, Ich or Marine Velvet, to provide easier respiration for the fish. The parasites are deeply embedded in the fish's body, but the gill area is much shallower, and the parasites there have little chance to surivive. To fully cure a fish, you must take either one of the two treatment options, hypo or copper.