Does this girl only text me whenever she has no one to talk to or does she text me everytime she is free?

I have been texting this girl online, and sometimes she takes hours to respond, but sometimes she responds instantly. How can I know if she is responding everytime she is free, or only responding when no one else is talking to her?
Does this girl only text me whenever she has no one to talk to or does she text me everytime she is free?
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