You can only experiment to determine for yourself. Remember, the issue isn't so much with plain chlorine, it's with the fact that most municipalities, especially if they're pulling (above) ground water, rather than from a well source, are going to now be mandated to use chloramine, a much more stable compound that does not "decay" anywhere nearly as easily as chlorine. I'm doing some new "research" into this, and it may be a good topic of discussion for organic growers, as chloramine seems to be a very interesting compound, especially from an aquatic perspective.
You should know that your trashcan, while chemically inert (if U.S. plastic, NOT Chinese plastic or metal) otherwise and GREAT for doing things like mixing up saltwater, does not offer a very good surface area-to-volume ratio (low surface area to total volume), as such, that prevents chlorine from outgassing effectively. To that end I suggest some means by which to aerate the water, which equals water movement. Aquarium air pumps are a cheap means by which to do this, remember, surface turbulence also equals best CO2/O2 exchange, which equals best outgassing for chlorine IF that's all your local water is treated with. I tend to doubt it, though. I say treat it like it's chloramine.
Now, here I am doing a little research, and from what I'm finding there is a very good chance your water supply is being disinfected with chloramine, not simple chlorine. Sodium thiosulfate, in this case, is your friend. It can be purchased in ridiculous bulk for ridiculously cheap prices from TheChemistryStore.com (search Google, best price sodium thiosulfate, et voila). Last time I checked you could get a FIVE POUND bucket for $10. Do you have any idea how much dechlorinator a 5lb. bucket of sodium thiosulfate makes up, even at an unheard of 5% solution???? God, I fucking love that stuff. Google it, it's got many, many uses, and it's blue. Not blue like methylene blue, which is REALLY blue, but blue enough as it is.
Linky linky!
The Skeptical Aquarist on Chlorine and Chloramines--READ ME
Man, all the weird things I learned during my fish & exotics years...