It sounds like you might be thinking of a different option. You have the "repeat every..." option selected, which will repeat the same notification to you every 30 minutes (or whatever you configure) until the condition is no longer true. If you uncheck that, you'll see an "Only once within this number of minutes..." option instead:
This sounds like what you want: you'll get at most one notification every 30 minutes, even if the sensor sends a new reading within that timeframe that meets your criteria for notification. Without either of these options, you'll get a notification every time the sensor sends a reading that meets your configuration. It is apparently not possible to combine them and get a repeated notification but not more than one notification within the time period.
If that's not the behavior you want, you could use Rule Machine to create a custom notification with whatever logic you want, but you'll have to decide if the tradeoff of using Rule Machine to get that power is worth the effort of either learning or building a rule.
