How often should an eye exam be done?

i used to slack on eye exams. there was one time i went 4 years without one. probably not a good idea. i have prescription gles. my last exam was February 2007. i thought i heard its best to get them checked out every year. especially since my mom said there has been Glaucoma in her family. i read about that and i have never noticed any of the signs they listed. is it about time to get one?