A couple of important things when doing a Fresh Water Dip:
Make sure the temperature is the same as the display tank.
Make sure the PH is buffered up to the same level as the display tank.
After doing that, the dip should last about 2-5 minutes max, or until the fish starts looking stressed. Ie. Gasping, floating on the surface, laying on the bottom, etc. If that happens, get them out and back into the tank.
The Osmotic shock of Specific Gravity change is less stressful on the fish than the PH or temp shock that may occur.
The PH level of Freshwater 7-7.5 ish is 10x as acidic as normal SW levels at 8.3, and can eat the slime/mucus coat and some scale and gill structure right off of a SW fish, leaving it wide open to further infections, etc.