A day or two for chlorine and about two weeks for chloramine. If it is in the sun it will be a little less.

Vitamin C will remove it almost immediately.

Wow didn't know about the vitamin c. Thanks

I've been told that chloramine does not off-gas.

Tough to find info online to back up that statement. But I finally located this:

"And unlike chlorine, which dissipates when water sits for a few days,
chloramines may take weeks to disappear. While sunlight and aeration
help remove chloramines from water, allowing water to sit is not a
reliable method of chloramines removal.

"Two effective methods for removing chloramines include using a
chemical to neutralize chloramines or using a granular-activated carbon
filter"

The above and more is in the final section of info discussing municipal water disinfection: Look for the section header 'Chloramines Removal' on this page:

http://www.ianrpubs.unl.edu/pages/publicationD.jsp?publicationId=777

I ended up using vitiman c (ascorbic acid) to neutralize the chlorimine. I read that a municipality somewhere, I forget, had stated that 1000 mg would neutralize the chlorimine in an average bathtub. So I've used 1500 per barrel. My barrels are now setup and awaiting the coming rain. Thank you both for your help.

Why bother? Everyone on city water uses it through their hoses, sprinklers, irrigation systems without an issue.