Defining collapse, to me this means US balkanization.
IMO it will happen when the US gets a few large scale climate related disasters like the Texas snowstorm etc but over a dozen(more than 1 a month) going on continuously for about 3-5 years would be enough to destroy the country.
I believe it is a certainty because the US government is not capable of mobilizing resources to deal with these disasters, at the end of the day the capitalist class will not allow it. This already happens anyway, Puerto Rico got fucked and Trump withheld resources, nothing was done anyway. You'll be told to leave or flee or get fucked. Everyone will cheer when Florida goes underwater but will they deal with the consequences of mass migration etc? You can expect headlines like this in the future Trump complains to senators that Puerto Rico is getting too much hurricane relief funding If this isn't a sign of collapse than what is it? The "richest" nation in the world counting pennies for disaster relief while e.g giving unlimited no questions asked funding for the military.
Anyway as for the timeline, well climate change is showing signs of nothing except getting worse, nothing is being done and nothing will be done until it is too late, you can look into solar reflection is the goto example, I am all but convinced the US will do it unilaterally and fuck up everything because literaly better to gamble the fate of the world than to talk about changing capitalism, degrowth etc.
So if you want a date? It is hard to say, we are on the very pessimistic path, you can look at all the depressing headlines if you want confirmation, everything is always "faster than expected" or "scientists shocked" etc even accounting for the usual MSM sensationalism this message is even stronger among the academic circles, before 2050 is already quite likely, but almost certainly "collapse" will happen before 2080.
Climate change dictates this will happen and to argue against it like saying its "cope" and whatnot would be saying that a country going through multiple disasters and mass migrations has not collapsed yet because technically there are still 50 states and some geriatric dipshit 80 year in sitting on a table somewhere being called a "president".