Hi Vader,

Antibiotics such as Vibramycin, Vibramycin 50, Vibramycin -D have a reasonable shelf life of around 4 years if stored below 25 C.

http://emc.medicines.org.uk/emc/assets/c/html/DisplayDoc.asp?DocumentID=1475#SHELF_LIFE

So I guess it depends on which antibiotic you have in your PSK.