She can only stay for maximum 90 days within any 180 day period, starting on the date of entry in the Schengen zone.
There are countries in Europe which don't belong yo Schengen, like UK or Ukraine, so she could spend there remaining 275 days.
She can also get permit to stay for longer on base of work. As a Canadian she should be able to get teaching job somewhere in Europe.
How would things work if we married - would she be required to have a work visa then or could she just be allowed to stay with me because of being my wife?
Then she would be treated by all EU countries as equal. Meaning permit for staying and working is automatically granted, but only as long as she lives with you.
I've heard of the 3 or 5 years rules in order to get permanent residence or citizenship.
Without marriage it's 5 years of continues, legal, not interrupted by more than 6 months, stay in Poland or other EU country. Study counts as 50%.
Would we be allowed to travel to schengen countries during those years or would my girlfriend/future wife need to spend a significant amount of time inside poland's borders specifically?
Girlfriend can travel, but max 6 months in 5 years.
There is one more option if you're not fixed on Poland and don't want to marry. In Belgium and perhaps some other countries there is kind of Civil Partnership agreement, which allows girlfriend to be treated as married by Belgian law, meaning it allows spouse to work, stay in Belgium and do taxation together. Problem it's not recognized by must of EU countries, so when leaving Belgium you will be not married :)