The Parkinson’s Law is a good way of thinking about how much cash to keep in the portfolio any time. If you have any portion of your portfolio in cash, it is extremely tempting to allocate it all to some seemingly good opportunities.
Good opportunities in investing are rare. If either the business prospects or the price are not compelling enough, it wouldn’t be prudent to buy just because cash is available. In fact, this method may be the only sure way to long term investing success. When you compromise on either business quality or price, you risk capital loss or miss some better opportunity.
So while it is very difficult to come up with an strict rule, it would be prudent to keep buying equity until the price seems well below an estimate of fair value. Most other things aren’t really under one’s control. Even trying to predict the direction of the markets based on valuation, or the current attitudes of other market participants — these all will eventually lead to one being wrong about predictions, and causing further mistakes.
Give priority to allocation, and keep buying equity slowly if the price makes sense. Don’t wait for a bear market, and don’t be too quick to buy on dips.