Each approved extension request will allow you an additional 6 months to submit your Statement of Use form.
You can submit up to 5 extension requests after the USPTO issues your Notice of Allowance (a temporary, conditional approval of the trademark application).
Including the 6-month grace period, this provides you with a total of 36 months from the issuance of your Notice of Allowance within which you can submit your Statement of Use. To view the date of your Notice of Allowance, visit http://portal.uspto.gov/external/portal/tow
and enter your trademark's USPTO serial number.
LegalZoom can help you file extension requests.