If you have sudden color shifts in your prints or blacks start to have a bronzed look or you have lines that run the same direction as the print head you should run a head clean. The head cleaning cycle ensures all ink nozzles are clear and free of any obstructions. Always run a head cleaning cycle if your printer has been sitting for more than a month between printings.
Head cleaning uses ink to clear any blockages from the print head so you don't want to be performing these unnecessarily. If there doesn't appear to be any improvement after the first clean, wait at least half an hour (longer is better) before performing another head clean. This will allow any blocked ink to soften and be removed more easily with the next clean. Don't perform more than 3 head cleans in a 24 hour period. Performing several head cleans in a row or more than 3 in a 24 hour period is like flooding your engine and will be ineffective and could potentially make the problem worse.