It really shouldn't be that bad. At worst it'll be moldy, but it's entirely possible you'll come back to find it just kind of shriveled and very squishy. It depends on how ripe it was when you left, and how warm and humid your apartment is. But even if it smells, it won't be the kind of smell that hangs around for very long after you throw it out.
Of all the food you could have left out and forgot about you really lucked out with it being an avocado. If it were milk or raw fish you'd probably be coming home to something truly horrific.
