How long is the life of rust monsters?
How long will a rust monster grow up from a child in the barrel?
According to the Ecology of the Rust Monster feature from Dragon Magazine #346, it takes about two years for a newly hatched rust monster to grow to its adult size:
After approximately a month the eggs hatch, each spilling out a tiny larval rust monster about the length of a man's forearm. These young have no appendages and look like nothing more than bloated, brownish maggots with moist antennae approximately a foot long. These antennae possess the same corrosive effects as an adult rust monster's, and over the next two years grow in size, along with legs, a bony tail, and a hardened carapace.
The article doesn't specify exactly how long a rust monster can live after reaching adulthood, but it's at least "decades." The feature details a particularly old, large, and powerful rust monster called o-Akasabi-sama, whose description includes the following:
Over the decades this monstrosity has destroyed the weapons and holy katanas of numerous master swordsmen...