When you sign up to OVO you are automatically signed up to recharge every 30 days - or every 365 days if you're on OVO Data - 365. That means that every 30 days (or as specified) your data allowance and any other benefits are renewed. If you don't want your recharge to happen automatically you can turn that option off (just go to your OVO Dashboard) - it just means that each month you'll need to remember to recharge manually. You can manually recharge and check on your benefits anytime on your OVO Dashboard.
If you run out of data during the month you'll need to top up. And, if you're finding yourself topping up regularly you can always change to another OVO plan. There's absolutely no cost for changing up (or down) - but it's best if you line that up with your next recharge otherwise you'll lose any unused data.