There are multiple periods throughout US history where the markets dropped and stayed down for lengthy periods of time. Consider this chart, for example:
It's often said "don't have any money in the market that you need within 10 years". That's IMHO very good advice. (Just look at the market returns from peak to recovery in the 2000s. There were two big drops during that decade: the dot com crash of March 2000 through Oct 2002 (-44.11%) and the Subprime Crisis of Nov 2007 through March 2009 (-50.89%). A $1M portfolio of 100% US equities on 1/1/2000 would have only recovered to $1,150,588 at the end of that decade, for a CAGR of 1.2% or inflation adjusted CAGR of -1.21%..worse, the investor would have spent 5 years and 8 months "underwater" (ie: value less than what they started with) from Sep 2000 until April 2006..
Generally speaking, it seems many people think that recoveries from bear markets happen 'quickly'. 2020 was, of course, an extreme instance of that but a once in a lifetime anomaly that was happened primarily due to stimulus. But taking more 'normal' market volatility into account (like the period from 2000-2010), it's quite possible to be "underwater" on total equity returns for 5-10 (or more) years. 1-2 year recoveries are definitely not the norm.
Now, for anyone with 10+ year investing horizons..that's a different scenario than those in retirement or with shorter time horizons where access to $$ is needed within that window..