hello, I am trying to decide on a new plant to grow and am having a hard time trying to figure out how long i would have to let desmanthus illioensis grow before harvesting? Thanks.

Given good growing conditions and care...

First year might yield a few seeds, the root would be laughably small.
Second year should produce lots of seeds, the root would be harvestable only with many plants.
Third+ year abundant seed production, the root should continue to rapidly increase in size until slowing down around year 5.

Climate and care can make a sizable difference but those numbers should be roughly accurate.