For your girlfriend to immigrate to Canada as your partner, you must either be married or have lived together for a minimum of one full year (common law).
If she is interested in immigrating independently (e.g. as a skilled worker), she should read up on the various immigration routes available here:
http://www.cic.gc.ca/english/immigrate/index.asp
How long she is allowed to stay when she comes for a visit to Canada is actually up to the immigration official at the border. Visitors from the US are typically allowed to visit for six months. However sometimes they are only allowed in for shorter visits or even denied entry completely. When she comes for a visit, she should bring her luggage to a minimum (i.e. she should look like she is coming for a visit - not to live). If possible, she should have a return ticket and she should be able to show enough money in the bank to support herself while she is here. You can request an extension to her visit once she is here.
