4 weeks minimum for observation if all you are trying to do is ensure general health.
if you want to ensure it has no more Ich on it, then you will have to treat it with TTM or Copper/CP first. if you have never treated other fish before, then your DT has Ich already as well anyway. so unless you have the time space and patience for a full out QT of all fish and run your DT's fallow for 72 days, then you may as well just do the 4 weeks observation. Ich will likely stay on the fish in the gills when moved over, just in a small number.