I usually don't change them until I hear funny sounds, like wear indicators or rivots grinding, or if I feel the shaking of warped rotors. I also tend to change pads and rotors more often than just pads. If I were planning a big trip, I would inspect the wear level to estimate if they would last through that trip or not.
A & A parents of DD 2005, DS1 2007, DS2 2009 2011 Suburban 2500 6.0L 3.73 pulling 2011 Heartland North Trail 28BRS 2012 VW Passat TDI
No worries, you should be rotating tires periodically and when you do that, you always check the brakes.... check both the inside and outside pads on the front brakes, they can wear uneven if the calipers stick at all... depending on the truck, it may be just as simple to check the rears, or it may be a bigger job.
YMMV depending on the driver, location. Are you a 2 footer or single foot driver?
Are your trailer brakes and controller adjusted correctly? A weak braking trailer will be taxing on your TV brakes. As said when rotating tires its very easy to view your pads. Or mr goodwrench will do it for you.
Brake life is inversely proportional to brake use. Therefore, if you never use the brakes, you can drive them forever. The more you use them, the more they will wear. There is no hard and fast time frame in which you should replace them. Simply replace them when they're worn out or their performance is no longer satisfactory.