When you log into your account, you will see the date when your next top-up is due.
Topping up early
If you top-up by £10 that date will be extended by 120 days, regardless of when you top-up. So if your next top-up date is showing as being in 40 days time and you top-up today by £10, your next top-up due date will change to be in 160 days time.
Topping up with larger amounts
You can also top-up by larger amounts and this will further extend the time to your next top-up due date.
When you top-up by £10 your next top-up due date is extended by 120 days, but if you top-up by £20, your next top-up due date is extended by 240 days, £30 will extend it by 360 days, etc