Hi There @
shashika,
Welcome to the country, 👋
To answer your question, any time you top up the amount of credit will reduce depending on your Usage and Appliances so the more energy you use the quicker the credit will expire the more energy efficient that you are the longer the credit will last,
When you have reached a zero balance you will need to top up again to get back on supply or access the emergency credit,
I hope this answers your question,
Kind Regards Dean