How long is too long to go without a contract?

Let's see...the year is 2007. The following local teacher unions in Michigan have been without contracts for:

One year:

Bridgeport teachers; contract expired August 31, 2006

Goodrich teachers contract expired July 31, 2006

West Branch/Rose City teachers; contract expired June 30, 2006

Two years:

Camden-Frontier teachers; contract expired June 30, 2005

Willow Run teachers; contract expired August 2005

Houghton Lake teachers; contract expired June 30, 2005

Three years:

Willow Run office personnel; no new contract since July 30, 2004

Willow Run paraprofessionals; no new contract since July 30, 2004