Hi! In my experience, they can be kept in the refrigerator for up to 1 year. Although it is desirable passages at least once in six months.

i stored my Klebsiella pneumoniae strains in -80 refrigerator more then a year. As Elena stated, additonal serial passage needed to check their lifespan

You can actually ovelay them with glycerol or mineral oil to extend their lifespan to months. You can also store them in ultralow freezer or liquid nitrogen,

For  the bacterial strains that are used daily or weekly, cultures grown on agar slants can be stored in a refrigerator assuming that precaution has been taken to avoid contamination, but the storage period will be less than 2 weeks.  Cultures should be prepared using standard techniques and then sealed before storing. Using screw capped tubes is preferred. 

Growth on suitable slants in tightly sealed screw capped tubes can be stored for up to six months in the refrigerator,  if overlayed with glycerol. Serial passage every three months is, however, advised.

For bacterial maintainance 750 microliterd of broth culture added to 250 micrometer of sterile glycerol in a caryo tube and kept at -80C

Dear Sony,

it depends on the temperature. At fridge temperature (some 4ºC) you can keep them for a couple of weeks in the case of Klebsiella, and probable less in the case of streptococci. At freezer temperature (between -20 and -80ºC) you can keep them for months. Only be careful with the tube you use for the slant. Glass tubes break easily at that temperature. Anyway, should you want to preserve for a long time your cultures, is better if you use the glycerol method suggested by Warda Abdelgadir.

Une colonie de 24heure sur disque de papier buvard puis inclure l'ensemble dans une ampoule slerile obturer et conserver à -20 degré celsius