hy Kishita !! 

First of all , I  would like to inform you that, for getting clerical jobs in bank , you need to be a graduate. Without a graduation degree you can not apply for bank clerical grade. If you are  12th pass out there are some lower level jobs for this category also. 

For these jobs , you should have at least 6 months of coaching. This much coaching is enough for clearing a bank examination. If you want to be very sure you can extend this time period till 1year which is more than sufficient for bank. 

If aiming for bank Po , then you must have coaching for 1 year as bank po is a bit difficult to crack. You should be at least 21 years ' for being eligible for BANK PO .

hope it helps!! 

Good luck !!