test at Kodak and Ilford have concluded that the quickest...most efficient... wash water temperature for prints is 80ºF. Lower temperature means longer washes. The time required to get a thorough wash is determined by the fixer used and the fixing time etc. Hypo clearing agent substantially reduces washing time. The cheapest way to make a hypo clearing agent is to use a 2% sodium sulfite solution...20 grams of sodium sulphite to a liter of water. A very good source of sodium sulfite at prices much lower than the photo chemical suppliers is The chemistry Store (wwww.thechemistrystore.com). This is also a good place to by things like glycol and TEA etc. As an aside one of the best places for fixers and developing agents is techcheminc...(www.techcheminc.com.) very nice people with very much cheaper prices , although less items being stocked, than Photographer's Formulary.
So "How much longer do you wash your prints". Buy a wash test from Photographer's Formulary. It is easily used with clear instructions and is the only way to answer with accuracy the completeness of your wash.