Your bundles are renewed the same day each month. If you started your subscription February 6th, your bundle is renewed March 6th, April 6th, etcetera. 

Used up your bundle, and don't want to wait for your bundle renewal? Change your price plan. Your new bundles are immediately added to your SIM card. If you change your price plan, the day your bundle is renewed also changes. If you change your price plan February 26th, for example, your bundle is renewed March 26th, April 26th, etcetera.