How much can you load on a 15 amp fuse?

Fuses can last for 10 years or more because fuses don't really wear out unless perhaps they are run continuously right at the edge of blowing. The constant heat of near-high amperage (which is what blows the fuse anyway) will eventually cause it to trip at a lower amperage than when it was new. Water will corrode a fuse and cause it to blow and low voltage on a circuit feeding a motor will drive the amps up, causing a fuse to blow when it would have been ok at normal voltage (12.6-13.6 dcv). And corrosion on the fuse blades inhibits voltage.