Where the truck is stored, what temp it's stored, humidity, how often you drive it, and how far you drive it makes a difference. I don't drive mine all that much, but when I do it's long enough to heat the oil and dry out any moisture, no short trips around the block. Mine also sets in a climate controlled garage, so the temp is steady with little humidity. I check it when I think about it, but see no reason to dump oil once a year with the way I drive the truck.
Get you some small clear bottles to take samples of your oil, keeping one for new oil. When the color starts changing dramatically, make the decision about changing it.
