Note the above is only for their equity portfolios. Benjamin Graham's take on broad diversification:
"We recommended that the investor divide his holdings between high-grade bonds and leading common stocks; that the proportion held in bonds be never less than 25% or more than 75%, with the converse being necessarily true for the common-stock component; that his simplest choice would be to maintain a 50–50 proportion between the two, with adjustments to restore the equality when market developments had disturbed it by as much as, say, 5%. As an alternative policy he might choose to reduce his common-stock component to 25% “if he felt the market was dangerously high,” and conversely to advance it toward the maximum of 75% “if he felt that a decline in stock prices was making them increasingly attractive.”"
I wouldn't recommend long-term fixed income at all right now, but this argument could instead be referenced using cash as a substitute. This is the tricky part for me as I think it's hard to gauge just how much to leave outside of equities in any given period.
Considering his financial assets, Buffett is currently half cash and half equities