It depends how often you use it.
When you apply, you need to deposit in your account enough for your estimated month's spend, then pay each month by direct debit.
Each time you use it, your tag account is debited. If you are running low on credit, the light flashes amber at you at the toll booth.
Any accumulated credit, over and above your needs, can be transferred back to your bank account. The account can be managed on line, so changes can be made quickly.