I searched Youtube and couldn't find any concrete answer. Most sites I found would give you a range of months when the avocado would be mature based on the variety, then advise you to pick one fruit, let it sit on the counter and if it's ripe then the rest on the tree are fine. If it become rubbery, wait another 2 weeks and try again.
The avocado has shinny skin, one of the forum member told me when the sheen on the skin is gone, it means the avocado is ready. I have my avocado on the tree since May, the forum member told me my avocado (Wurtz) would be ready in October/November, so I guess it's normal to leave fruit on tree for 5-6 months.