How long should a texting conversation with a girl last?

Sometimes we run out of things to say and the conversation gets boring so i end up making an excuse to leave so it dosnet drag out or anything. So whats the perfect amount of time to text a girl? 30 minutes, an Hour, or More? We both really like each other and we're both 16. Thanks! :)
How long should a texting conversation with a girl last?
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