Yes, the long term traders definitely "need to thread with caution" before the cycle changes, which it inevitably always does, without exception.
The margin debt goes up and down in cycles as well. Take for instance in October 2007, when the market peaked, the margin debt was slightly over $345 BILLION. It created a trough of almost 50% to around $173 BILLION by February 2009, when long term investors were BURIED after the financial crisis and the S&P had lost over 40%.
During the recent multi year bull market, the margin debt has slowly crept up again, surpassing the $500 BILLION mark twice in 2015 (April and June), and currently hovering in the mid $450 BILLION range.
How low the margin debt will go is anyone's guess, but it will surely drop again to a new trough, and long term investors will get BURIED, once again!
The margin calls are coming...
http://www.nyxdata.com/nysedata/asp/factbook/viewer_edition.asp?mode=tables&key=50&category=8