There are many parallels between drug resistant bacteria and herbicide resistant weeds, including population numbers and how resistance is gained and lost.

Since there isn't parallel gene flow in weeds, the answer hinges upon the question "does the resistance trait put the organism at a competitive disadvantage in the absence of the herbicide?"

If the answer is no, then the trait will persist for many generations, because there's no cost to keeping it, and plants like to keep many genetic pathways for handling threats - that's why their genome dwarfs ours.

If the answer is yes, then the trait will diminish at a rate dependent on how much a competitive disadvantage it puts them at relative to weeds without the trait.

So if it affects their water efficiency in a dry climate- it's going away quickly. But if the cost is minor, it will last longer.