I’ve been planting and distributing hundreds of avocado trees over the last couple years in the hope of eventually answering this question, but it’s too early to say.
I have the “Poncho” or “Pancho” cultivar growing outside for a couple years now, and the coldest it has survived without protection was about 18°F (minus 7.8°C). I protected it fully (with a cover and heat bulb) on the few occasions it’s been colder than that. Here is what it looked like in its first winter outside:
More recent photos of that tree are here on the project website.
I’ve tested many first, second, and third year seedlings outdoors, including some that were grafted. Such young trees have almost all been killed above ground when unprotected below around 19°F (-7.2°C), and some have been killed entirely. But many of the ones killed above ground have continued to get larger each year as they regrow from the roots. I’m hoping for a few milder years to let some trees size up.
The most recent post in my avocado project thread shows a grafted tree that survived 14.7°F (-9.6°C) with only a bit of mulch mounded above the graft union: