If your prepaid is for data, then you can set your monthly limit (in MB or GB) and specify your monthly expiry date. Just go to Settings > data sense > set limit, then choose "Monthly" under limit type and fill-in the relevant information. But if your prepaid does not necessarily put a monthly cap on your data use, then just set the monthly date and an unreasonably high data limit (like 1000 GB)*.
*- or if you live in a country like where I'm in where "unlimited data" means 1GB max per day (yes, it is ironic), then I'll place 30GB in my monthly limit from 1GB per day multiplied by 30 days per month.