Yes, the USSLC allows you to submit a new background check once you are within 6 months of your current check expiring. However, keep in mind the new check will be good for 24 months from date of submission, NOT 24 months from the current check's expiration. If your check expires in November and you submit a new check in August, the new check will expire 24 months from August (meaning you will have "wasted" 3 months of the original check).
MNSRC recommends renewing your background check 3-4 weeks before expiration.