Scrum Alliance® certifications are renewed on a two-year basis. If your certification has expired, you're still able to renew it successfully after the expiration date without a grace period.
At this time, your certification will never be revoked as a result of missing your renewal. If you renew past the expiration date, your certification will be extended for a full two years from the date that you pay to renew (vs. from the original expiration date.)