I have heard it could be 6 months to a year. That's why we're replacing on our own and expect to be paid later.
I will say we have seen improvement on some of our trees, especially arborvitae--but the Jap Maple is clearly dead, although it's hanging on to curly, dried up, reddish leaves. I planted a crab apple that's doing fine; Our Norway Spruce (one died early on), still grow, but not from the top and are distorted on the sides. We're told we 'could' train new leaders...(no thanks). We replaced a very large locust 1 year after spraying--it has several dead branches, mostly at the bottom, but otherwise looks very healthy and we'll keep the tree.