Dear,
As far as my knowledge goes, there is no mention of the time duration for minimum funds on the CIC website, www.cic.gc.ca
All u need to do is posess the minimum amount of unencumbered liquid funds readily transferable to canada in your name. The money available should be yours own and not borrowed one. If you can provide a current bank certification about the funds and maintain your balance in the account throughout the immigration visa process and later on carry the specified funds to canada at the time of landing...then i feel that age of bank statement is of little value !! So go ahead and apply..wish u all the best..
Pls correct me someone with a link if you have alternate source of information