How long does antibiotic-dosed LB maintain good selection? Various people in our lab will prepare a liter or so of LB, add kanamycin to 25-37 mg/L for selection, and store it at 4 °C for minipreps or other small cultures (where dosing straight LB with a 1000X stock is troublesome). Some think using it after more than a week is dubious, but we routinely use kan plates that are 1-2 months old with no ill effect. How long can LB with antibiotic such as kanamycin, chloramphenicol, or ampicillin be stored at 4 °C and maintain 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.