I think you can try freeze drying process.

Are you talking about a continuous culture of the same bacterial strain? At room temperature it should be as simple as re-streaking colonies onto fresh plates every day or two. Of course, that's assuming you're working with a common species (like E. coli), a strain of that species which grows well at 20 degrees C, and don't wait so long that you select for a GASP phenotype like that described in the paper linked below.

http://femsle.oxfordjournals.org/content/266/1/119.long

I do not understand what you mean by life ? If it means viability, then you could follow any of the following preservation methods 

Reference Murray`s manual of clinical microbiology Chapter 9 volume 1

Freeze drying methods

Spray drying methods

Another method is inoculating onto an agar slope/slant or stabbing into an agar and incubate, then keep in a cupboard in cool dry place not exposed to sunlight. But note this works well for hardy bacteria like enterobacteria e.g E. coli NOT fastidious/fragile bacteria. Therefore, the question is what is the name of your bacteria???

Thanks for your comments. I wanted to know how to extend the life of bacteria in culture not as a colony like we store the bacteria which are used as bio-fertilizers or other similar use. How we can extend their life?

try a chemostat or turbididstat

I could not understand the question properly.  If I am correct, like bioinoculants if you want to keep a microbial cells as formulated product with a good shelf life to be used for agriculture, you can do it with liquid formulations.  Grow the organism in respective medium up to mid log phase, harvest (by centrifugation), wash it and resuspend in a minimal medium with proper cfu/ml  (there are n number of media available - standardize for your culture); add a cell protectant (glycerol or trahalose or polyvinyl pyrolidone).  The  shelf life for this will be 1 year (Will have 8-10 log per ml if you start with 12 log per ml). Good luck.

Balu

I think, its depend on your bacteria species. For common bacteria like E. coli, it will be really difficult to do. We commonly employing freeze drying it to keep the viability, even some time it reduce total log bacteria. I though if it's actinobacteria, it will be more long period to keep in room temperature, but i'm worry it will not survive for 1-2 year. 

Yes you can; for Gram negative either you go for liquid formulation containing additive/osmoprotectant as recommended by Balu or induce cyst formation. I personally have experience of saving Klebsiella for more than a year only in 0.85 % NaCl solution after growing it in LB broth. You can also try oil-water emulsion at room temperature. While for Gram positive bacteria grow them in suitable media and apply broth to some powder based inert carrier, dry it, u can save your bacteria for 2 years by this method. Spray drying is also one solution but I think you want to go for simple solution. find attached files for further detail.

This review may also be helpful

https://www.researchgate.net/publication/259104026_Advances_in_plant_growth-promoting_bacterial_inoculant_technology-_formulations_and_practical_perspectives_%281998-2013%29

Article Advances in plant growth-promoting bacterial inoculant techn...