The question isn't how long but where have you let the avocado during this time... In some place it will smell bad very quickly ദ്ദി(˵ •̀ ᴗ - ˵ ) ✧

Most fruits and veggies IME tend to dry out more than rot, unless perhaps you live in a very humid area. My experience with forgotten avocados is that they'll shrink and get wrinkled, while becoming inedible inside, but won't be problematic.

Tomatoes can definitely get gross and leaky if forgotten. Potatoes when rotting can actually produce poisonous gas, so watch out for that. Kind of a Solanaceae (Nightshade) theme there, I guess.

Probably will be fine, avocado skin is very tough.

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.