This particular chemical degrades over time, and more quickly by being dug into ground than being stacked (soil microbes or something like that acts on it) but I'm not sure off the top of my head what the particular timescale is.
The timescale has not been established. Dow claim a half-life of 35 days, but on our site we have seen sensitive crops such as potatoes and beans still showing the effects after the stuff has been in cultivated ground for over a year. The United States Environmental Protection Agency in their tests on aminopyralid found a range of half lives in samples soils that they tested which ranged from 31.5 days to 533.2 days
http://www.epa.gov/opprd001/factsheets/aminopyralidEFEDRA.pdf page 14. Also, seepage through the top layers of soil from a heap of contaminated manure has ruined my own greenhouse tomatoes. The stuff is highly dangerous to some of the most basic and necessary crops we grow as allotment holders and is definitely more persistent than Dow claim.
Furthermore, if you grow brassicas using this manure, you will get a good clean crop (with less clubroot in our experience!) but don't put any of the leaves on your compost heap or indeed
any waste from crops grown using it. They must be disposed of in some other way.
The petition at
http://petitions.number10.gov.uk/aminopyralidban/ has now reached 1500. Please sign, contact your MP, anything to keep this persistent and secret killer off the land.