There have been periods of time when the market would have tested the patience of anyone subscribing to the buy and hold approach. The periods of market history I refer to are the 1929-1954 and the 1968 to 1982 time periods. The markets, as defined by the DIA Index, took 25 years to recover after the 1929 crash. And, the ~1968 to 1982 time period although a less severe bear in terms of depth (-45 % compared to the -89 % loss of 1929) but it was a time period where the market went sideways for about 15 years.
It is easy to show that for these two time periods, a lump sum, that was invested at the pre-bear peak, would have performed much better had it been in a nominal rate GIC. The green lines on the graph of the DJIA show a few recovery times, including the two mentioned above.