On my Jeep I change the plugs every 25,000 miles. If I try to push it to 30,000 miles, the engine gets pretty unhappy. On that vehicle I use standard copper or platinum plugs. Nothing fancy because of the high change interval.
On our Escape - I'll probably change the plugs every 50,000 miles. I know that the owners manual says you can go longer but I just have to tinker all the time.
2009 Ford Escape XLT
4 Cylinder & Automatic
Pretty Baisc little truck. Only options are Sun Roof and Sync
First Mod: Spash Guards