Honestly to Primo’s credit he was one tough SOB who always got up even against the most detrimental punchers of the era like Louis and Baer. He could probably get up until a TKO is called, likely around rounds 4-6
In the 4 years before the Ali fight Foreman took out 15 of 17 in 2 rounds and at that rate him finishing someone in 2 rounds is pretty reliable. But I'll give Carnera at least 3 or 4 because how big he is and because Louis and Max Baer at their respective bests took 6 and 11 rounds to finish Carnera. Theres not many comps for 1973 Foreman but 1935 Louis and 1934 Max Baer are 2 of the closer ones. It will take longer than normal. Against Max Baer(the absolute best version) Carnera rallied to win rounds after taking SIX knockdowns in the first 3 rounds. He wasn't just hovering in and out of consciousness he still had the ability to meaningfully resist. I'm not saying Foreman isn't capable of taking Carnera in 2 but I'd expect him to last longer. Uzcudins known more as the "chin guy" and not getting knocked down>surviving 9 knockdowns but Carnera was incredibly durable. To clarify Louis wasn't his best when he fought Carnera but he was the most overpowering at that time in terms of destroying people.
