Answer

Some strains of bacteria can be kept in a fridge for between 1 and 3 months, but most will not last this long.
In order to keep the culture alive for longer you will probably need to sub-culture you bacteria, which means either transferring some of the culture onto fresh nutrient agar plates or transferring some into nutrient broth which has been sterilised and cooled. This way, the culture should grow and multiply, until the nutrients in the agar or broth are exhausted. Before it reaches this stage you will need to sub-culture again and keep doing so until you come to use them.

The link below is to a website which has details of how to sub-culture bacteria and other microbes.

https://www.scribd.com/document/317121056/Subculturing-Techniques