How long to stay in a first job before moving on

I [22F] graduated from college this past May in Econ and work in Finance. I started my job in June so it's been 7.5 months and I hate hate hate my job - I am never given anything to do and I'm very miserable. I want to apply for new jobs, but I'm worried it'll look bad on my resume that I've had my current position for so short of a time. What is a good way to explain why I'm looking for a new job in interviews and when can I start looking?

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