It will be fine for a number of weeks in a plastic bag. You might consider double-bagging if you're an infrequent smoker, maybe a third bag of stuff to smoke that day/week/whatever. Glass jars are pretty cheap though, and you should get some of those if you plan to continue smoking somewhat regularly (talking about like $1-3).

While plastic bags will work fine, glass jars (or something like that) would be better. To address what you actually asked.. your herb should not lose potency from plastic bags for a few weeks. Technically, the static of the plastic can pull off some of the 'magic' but it's not a big deal on small timescales...

I usually save all my plastic bags that have held herb for a while for when I make some hash oil or something...