HMD Mobile plans works on a 30-day cycle. This means that every 30 days, your chosen plan will automatically renew. You will be charged 2 days before your next 30-day cycle begins.
If you buy a plan for 90 days, you are essentially paying upfront for three 30-day cycles. After those 90 days, your plan will automatically renew every 30 days. You can see the next plans in your queue in HMD Mobile account.
Unless advertised, your mobile data allowance is valid only for that 30-day cycle. At the start of the next 30-day cycle, your data allowance will renew according to your chosen plan. Unused data allowance does not carry over to the next 30-day cycle.