With the latest monthly margin debt stats from FINRA just out it's worth taking a look the chart of aggregated stock market leverage. The chart shows the combination of net margin debt, net leveraged ETF AUM, and net speculative futures positioning, all in nominal terms, and the sum total is about US$400 billion. 

Standardized against S&P500 market capitalization, this represents 1.7% of market cap - which is the highest reading on record (even higher than the dot com bubble peak where it was 1.1% at the top). The issue is when this type of indicator reaches a very high level, it can still technically go higher. Rather the things to watch for from a timing standpoint are when leverage is either accelerating too quickly (it isn't) or rolling over (it isn't doing that either).

So while it's definitely at lofty levels, for now it is more of a background feature... similar to high valuations.