Anyone who knows how long it takes for the predator mites to overwhelm the spidermites and start taking control please let me know

They start taking control in just a few days, depending on how bad your infestation is. However that's all they will ever do. Do you want to keep battling mites for the rest of your weed farming venture? If so go ahead and waste your money on preditor mites. Otherwise, why don't you bother to search before you waste everyones time. You would have found the only real answer to this is in my reply to someones post which I think was entitled "spider mites". I'm not going to tell you what the solution is, but it costs about $8, and takes about 30 seconds, and will rid your grow of mites forever. You may not believe it. Niether did I until I tried it. I battled them for years and tried everything except bug bombs.