A couple of weeks isn't too bad and it's going to depend on the brand you buy, the exact capacity etc. Fact is however, leave mags loaded too long and the springs will get f'd eventually, and it could happen sooner or it could happen later, but you've got no way to tell how soon (or late) it'll happen to your mags, so why take that risk for the sake of spending 30 seconds to pull the detent aside and unload the rounds back in to the bag/bottle?
You can do it with RS mags, those can be left loaded for literally 5-6+ years and work fine, but those springs are a totally different type to the ones in an airsoft mid-cap.