Manual says 15K intervals on the secondary fuel filter element (above the VC) and draining the water separator (if you still have one that works properly) every 5K. I've always stretched that filter to 30K without issue... with a mechanical fuel pressure gauge hooked for one interval as Icanfixall suggests. Imitate at your own risk, I suppose, because he's right about it being variable. Maybe I've been lucky over 25 years. I now have a 30 micron "water absorbing" Racor filter in place of the original water separator and Racor recommends 30K on that, so I'll likely sync those schedules and do 'em both at 30K.
Did get a bad load of fuel on a trip one time, decades ago, that set the "Water in Fuel" lite off. I had to stop about every 100 miles to drain water off when the lite kicked on. Finally went to a truck shop where they used a pump to suck the tank out (it was the front). They had a portable tank with a sight glass on it and of the 6-8 gallons of fuel in there, about a gallon of water separated at the bottom. We let it sit a half hour and then they pumped about 5 gallons of the fuel back in and I went on my merry way. The lite came on once or twice more on that trip but about 6 years later, that tank developed several rust pinholes in that little well at the bottom and I had to replace the tank. Always thought it was the little bit of leftover water from the previous "incident" that did it, whether that's accurate or not. Wanted to blame somebody!
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