The notion of the middle class, as we know it today, started around the late 1940s-mid 1950s. Historians and economists argue that up until the 1930s, a great majority of the US were poor and rural (poor is subjective here. Poor in the sense of how we perceive poor now is a bit different being a poor farmer living off your land in those days. Cant forget Great Depression and Dust Bowl either).
Our current middle class model, if we follow this trajectory, started from 1950s and peaked in 1980s, with a decline starting in the 1990s from gradual to extreme (40-50 years total up intil now). Currently, its showing signs of this “experiment” going awry. I'd say the middle class, as we know it, can seize to exist within 20 years if things go the way they are. If so, it would have lasted roughly 75-90 years max (that's being conservative). Human “society” began about 6000 years ago, and was relatively slave driven and/or serfdom/peasantry driven until the 1900s. With the middle class shattered, it leads humanity to regress back to this concept again. With property ownership becoming tougher and tougher over the decades and productivity demands going up while affordability down, we already see signs of the middle class (called the majority class since the 50s/60s) become indentured one way or another and dissolve into the lower class.
So, all in all- TLDR: 75-90 year long experiment (the middle class) is gone wrong and we could go back to serfdom and peasant class.