If you're worried about leaving a startup early because it's not the right fit for you, don't. Your life, your choice. Plain and simple. Suggestion: review the employment agreement you signed to make sure you won't be penalized for leaving early. If you received a signing bonus, you may be obligated to repay all/portion of it if you leave willingly before you complete your first year.

If the concern is applying for new jobs, then that will depend on the companies you're applying to. Startups are volatile by nature; they'll be more interested in what you did during your time than how long you were there. Traditional corporations or long-standing businesses however may weigh duration more as they consider you.

Best advice to you is to be prepared to explain your reasoning for leaving, and also demonstrate that you were effective in the time you were with your current employer.