You'd have 30 days to return the console, opened or not it doesn't matter, there isn't a restocking fee. You won't be able to return any games that you open though.
my xbox got rrod a couple weeks ago...sent it into microsoft for repair...thinking it would take a couple weeks till i get it back i went to best buy and bought a brand new xbox so i can play on it till i get mine back...turns out the turnaround for the repair was less than a week so i went back to best buy and returned the one i bought with no problems