Credit held in account

What is the recommended level of credit to be held in account? Early 2018 I was advised that I need one month’s payment to be held in credit. More recently (when price and monthly payment increase was notified), this became two months payment to be held in credit. This seems excessive. Can anyone advise?

From the terms

"3.21. If you would like a refund:

3.21.1. Upon request, we can refund credit in your Bulb account that exceeds your expected energy usage for the next month."