I heard Hillary was ready to leave him in the late 1980s but some of their close political friends/allies basically told her that Bill was going places but he couldn't get there without her. If that's true then that is the point where she decided she was staying with him regardless and I don't see her leaving after that. Especially since I imagine it was fairly early in his first term when she started thinking of running for office herself.
By the time he ran for president, they were stuck with each other.