Visitor visa and waiting for PR process

I am a Canadian citizen and my Korean girlfirend and I are moving in Canada this summer. She will be getting a TRV and we will marry while in Canada. After that, we'll be applying for PR status and beginning that whole process.

I have some questions about this, where I've gotten some mixed info on.

I've heard that the PR process can take between 12-18 months and that's also what I've read somewhere on Immigration's website. What do we do when her 6 month TRV visa is due to expire and we are still waiting for PR process to conclude? Can she extend based on the pending process? If so, how long can she extend her TRV for? Or will she need to leave the country and come back and get a new 6 month TRV?

Thanks a lot!