I personally would not ever make a non-refundable car or hotel as there are changes all the time. It could even be weather or illness, unrelated to SW schedule. That being said, SW does change their schedule. I think (but not sure) that when you receive an email that says "Your MM/DD trip to CITY, ST is all set" is when you can be comfortable they have set their schedule. I received this email for a 7/22 flight on 6/22 so about 1 month in advance.
