How long does antibiotic-dosed LB maintain good selection?
This Bitesize Bio article is very informative on this issue.  The 1970 study they cite found negligible reduction in the the efficacy of non-beta-lactam antibiotics (kanamycin, chloramphenicol) over spans of 4 weeks or 60 days.  Ampicillin loses about 10% activity at 4 weeks.

In my lab, we only take care not to use LB/Amp plates that are older then 4 weeks, for LB/Km, we store them at 4 °C for more then a month, sealed in a plastic bag.