This is not how it works. USCIS schedules you for appointments. You don't get to choose dates, only can ask to reschedule appointments (cannot guarantee it will be granted). It's unlikely you'd get interview for I-751 within 6-7 months of filing, but biometrics may be scheduled in this timeframe by USCIS.
Ideally, one should not leave the US until biometrics are reused / taken. If leaving before that, it's advisable to monitor USCIS account, case status and mail for updates. If appointment is scheduled, it's recommended to drop everything and come to the US for the appointment to ensure there is no denial of I-751.
Good luck!