My tobe wife is in UK work permit, question about dependent visa?

My girl friend and to be wife is in UK work permit. She has completed her bachelor's degree from UK and is now working in a renowned company under work permit visa of 5 years. We will be getting married in the next 6 months. I would like to know about the bank balance she needs to show for my visa application once we get married. I know about rest of the doents needed, but am confused about the bank balance. Can anyone please let me know about it. Thanks in advance.