You don't have to wait at all as far as the application goes. If you're not in any hurry, waiting for a few months will be less suspicious than getting married right away, but either case would be workable.
But ... a PR has to be in Canada to sponsor a spouse, and stay in Canada during the sponsorship process, so you would have to wait until you moved here to sponsor her.
Warning: If you and your girlfriend were living together for a year or more, she will be considered your common-law spouse for the purposes of immigration. Since you didn't declare her on your application, you won't be able to sponsor her. If this is the case, don't admit on her application that you were living together.