Minimum Time-Period of Bank Statement for Visit Visa????????

Hi All,

I am seeking an advice/help from all the members and GURU that I am going to apply for Visit Visa for my Mother from Pakistan and currently I am on Tier-1 (G) status with my family here. Minimum for how many months, I need to show the Bank Statement for this purpose and what should be the minimum balance for this whole period.??
We are three here in the family altogether.
Can anyone else plz provide me the list of documents i need to send from here to be attached with the application form.


Kind regards and thanks for spending the time.