if you are using a rinsing solution (highly recommended) then follow the instructions (usually 2-3 days). It also comes in handy for final flushing before harvest.
If you are using straight tap/RO water then it will take longer - like about a week to completely purge the nutrients (though this isn't necessary). A day or two with or without rinsing solution should get your PPM back down to acceptable levels. I would drain and refill the res.
bugsbunny is correct - don't look for damage to be repaired - look at new growth to determine if the problem is resolved but 6.0 is kind of high for hydro -- shoot for 5.5-5.7pH when growing hydroponically.