The last time Hillary was on the cover was in 2000, when she won election to the Senate. Hillary's had higher quality covers than other Clintons, too, being more likely to get a full solo feature, in classic First Lady tradition. Here's the first time she made a cover by herself, from January 1993. (She was also on the cover in November 1992, with Tipper Gore.)
Chelsea didn't get her first solo cover without another family member until 2010, when she got married.
(She and her mother appeared on a cover in February 1999 pegged to the Lewinsky scandal, titled, "Grace Under Fire." It was poorly received.)
And Bill? Well, his first People cover without any other Clintons was a bit more ignominious.