Monthly Data will only expire at the end of the NEXT month. This means that if you haven't used all of your monthly data by the end of the month, it will rollover to the next month. For example, if you get 10GB per month and you have 3GB left at the end of February, this 3GB data will rollover to March, giving you a total of 13GB to use until the end of March.
All Data Topups purchased only expire at the end of the following month. For example, if you buy a data topup on 5 February, your topup data will only expire at midnight on 31 March.