I have and 1.5 acre pond of which 1/3 of it has hydrila around the shoreline. I am stocking ten young grass carp this week. About how long should it take these guys to get it under control?

From everything that I have read, if you get results quickly, they will remove all vegetation from your pond which is typically a bad thing. If you added the right number, it will likely take a few years to reach the right balance. Ten seems high to me, but my climate is quite different from your's. Hopefully this is a recommendation from someone that is not selling you fish.

it was from a guy who has helped me with my pond for quite awhile - a pond management company

if they at some point are cleaning it out too much I will dispatch them -

That is definitely a viable alternative

Be very careful acclimating them in this hot water