Every Bank has an internal complaint resolution matrix.
General it is 30 days. 10 days at branch level and then 20 days with the nodal officers. You can ask for complaint register at Bank to register your complaint. It is mandatory for the banks to display their complaint escalation matrix on the notice board and on their website. There is specific tat within which they have to respond to your queries. In case you do not get satisfactory resolution to your query you may raise it to banking ombudsman.
It is adviced to share all your written communication to RBI for a faster resolution.
And yes, have witnessed RBI's assistance in complaint resolution wherever the complaints are valid and reasonable.