Water in Fuel? When Do You Drain the Filter?

At work we got 4 new F-450 utility buckets trucks at the same time. About 250 miles on the one I was driving and the light comes on. Called the company mechanic, he comes out and drains it, light never came back on. In a short period of time the other 3 new trucks had their light come on. Drained and good to go. Turns out the upfitter left them sit out in their prep lot with no fuel tank caps on. My 1995 Ram, the light came on once while traveling on I-81 in Virginia. Pulled over, drained it, the light goes off. About 100 miles down the road it comes on again. Pulled into a truck stop, got an aluminum baking pan out and slid it under the drain hose. Nope no water in the fuel. Got to where we were going, put the ohm meter on the sensor, it was intermittently going south. Went to a dealer close by and got a new sensor and all was good for 146,000 miles when I sold it. Our 2003 Dually which we bought new and still have, has 261,000 on the clock and the light has never come on. I change the fuel filter about every 15,000 miles. Alaska, Canada, US, no problems.