First off, know this: There is no "they" when it comes to weed. Buying weed is not like buying something from walmart. One persons definition of "sprayed" will always be different than others. If a dealer buys a pound of grass, sits on it for too long and it drys out, he will sometimes spray it with water to get it back to weight before he resells it.
Truth is though, you never know who has touched, pissed on, defecated in, or sprayed god knows what on your commercial weed. I would say that 75% of the commercial weed sold in the US has had no more than water, if anything at all sprayed on it. Its not like the farmer is spraying shit on his crop to make it smell or look good, because when it leaves his hands it looks & smells fine without having to do anything.
Once it sits for a long time, and mind you any bag you buy could be YEARS old, it will dry and smell funky. Also, its not a complete myth that Mexicans piss on weed before sending it to the states, because they know most of it will end up in punk ass American kids' hands.
People who claim a bag has been "sprayed" or "cut" or claim that they know exactly how it was modified just by looking at it are usually 100% full of shit. Take your chances or grow your own, plain & simple.
As far as the weed getting you stoned for 2 hours......is this a problem? Is it a different high than usual? Chances are if it was laced it would A) be more expensive and B) give a totally different high than just weed.
KB