So I haven't tried actual UHT; I use 'ultra-pasteurised' organic milk, which is refrigerated, but has a much longer expiry date than you would expect from milk, and you can taste the extra heat treatment. 8 hours works great and gives a thick yoghurt (haven't tried less than that; usually do overnight so around 12 hours). It does tend to get progressively less thick generation after generation. I use a supermarket organic plain yoghurt as a starter (there's a certain ratio of starter yoghurt to milk required - I use 4 Tbsp yoghurt to 1L milk)
What I don't understand is: Why does heating + 6.5 hours work for non-UHT milk but not for UHT? It's not just me - I Googled at the time and many others had the same experience: UHT milk often fails. Like me, they all were doing 6-8 hours.
Googling now, I see lots of people arguing if UHT makes it harder to make yogurt or not. Personally, I'd like to see examples of people making decently thick yogurt (without straining) in under 7 hours with UHT (whether they scald or not).
This site[1] says there was a study done that showed unheated UHT was runnier than heated non-UHT. I didn't check the study to see how long they set it for.
This site[2] also points out that UHT for 10 hours was still quite runny. They had to add powdered milk to get it thicker.
[1] https://www.healwithfood.org/recipes/uht-milk-homemade-yogur...
[2] https://www.everynookandcranny.net/instant-pot-uht-milk-yogu...