I have been riding on the Berkshire train for 25 plus years, holding through ups and downs. It has always been a huge percentage of my total investments, and buying over the years on selloffs and holding has resulted in returns about 3.5% above the S&P 500. The only shares sold during this time were those required after recently placing our first taxable shares purchased in a charitable remainder trust.
In the past few weeks, I've been selling off Berkshire in IRA accounts. Lowest cost basis $32.xx. Is it an attempt at market timing? To be honest with myself, I would have to answer yes, remembering the period between early 2008 and 2009. But now gliding through my 7th decade and financially secure, I no longer feel the need to be fully invested. Is Berkshire ever likely to be priced at 2x book again? Who can say? But My guess is 1.3 is more likely.
Disclosure: I'm a mediocre trader as proven over a long time period.