When harvesting agave nectar, generally the whole plant is harvested at once to get to the core, where most of the sap is. There isn't exactly an easy way to continually get nectar from the agave plant since it is herbaceous, whereas something like maple trees can be tapped because the wood can support the spigot.
Agave is more closely related to asparagus and hyacinths than to cactus. They are perrenial, you can harvest the lowest half of the leaves to make it look like a little palm tree, it will die after flowering, it takes 10-15 years to flower.
Agave are pretty tough. The way I propagate them is to cut the head off, and lay it on the ground, and come back in a year or so. I live in a semi arid climate. They grow faster if I water them every few weeks or so in the dry season.