Well, depends on where you live. What's your application status on the
New Citizenship status Dashboard and
ECAS ? If it still says "In Progress", then it means that the certificate could not be printed yet. If it says complete, the it is likely that the certificate was printed but not mailed yet.
Depends on where you live (how far you are from the office) too.
IRCC says it could take up to 4 weeks to receive your citizenship certificate.