Odds of Hanging On
Historians who actually secure employment will likely stay at their government agency, historical society, or museum for a long time. Those who do most of their work online with blogs and e-books will likely have to slog it out a bit harder.
The best odds are for those working in education (source). If you obtain tenure at a university, it's almost like becoming invincible—from getting fired, that is, so don't step out into any heavy traffic. Tenure, or life tenure, means that you cannot be terminated without just cause.
To get to this position, you've got to work your way through the ranks. You must publish research, teach for several years, and be highly visible at your school.
Basically, you're putting in a lot of time in an effort to convince people how smart you are. But as a historian, you'll get really used to that—people knowing how smart you are is just part of the job.