2013 F-350 6.7 diesel. I observe factory interval of 15K miles for filter change; 5K for separator drain. Ford does not indicate a percentage so I can't help there. I do have an aftermarket water separator/filter installed before the first factory filter. See
http://www.dieselsite.com/fuelsystems-cpr.aspx I replace the aftermarket filters at 5K intervals. As noted by others, biodiesel and unknown fuels when traveling can shorten the interval.