"It's probably an OK time for a hold of over 10 years, since any entry price now will seem good compared to the ending price. But the valuation levels right now are noticeably above the usual, so I'd expect a flattish year or two at some point. You'll almost certainly see better times to buy.

One possibility: Sit on cash for now. Sleep well. Buy the next time it's trading under 1.4 times book per share. (use the highest book per share seen to date, as it can dip temporarily in bear markets). My hunch is that this opportunity will happen soon enough that the entry price will be lower than today's price, and you'll be earning interest between now an then."

While I think everything you said is perfect. I would offer up a potential middle ground.

Maybe invest a decent chunk of the money now (if you plan to hold for at least 10 years), maybe 20-40%. Then make regular smaller periodic investments (maybe monthly, maybe a few times a year depending upon how much you have to invest), until the price/book ratio drops under 1.4 and then invest the rest.

By making regular periodic investments you smooth out the importance of the entry point in the eventual return. You won't get the best long-term returns, but you won't get the worst either and should do better than average over that timeframe. It also helps if BRK stays about 1.4 book for a real long period of time. Not likely, but not impossible either.