When does a Canadian corporation have to register for GST/HST?

Once its taxable sales, together with those of associated businesses, pass C$30,000 in a single calendar quarter or over the last four consecutive quarters. From then it is no longer a small supplier and has 29 days to register. Some provinces add their own sales tax, so we flag this when we review your business model (confirm with your specialist).

Source: When to register for and start charging the GST/HST, Canada Revenue Agency