The guys who change the oil say I need to have power steering fluid changed...how often should this be done?

Help! My check engine light came on (11,000 miles past warranty), mechanic said the code was P2716. Also noticed it was shifting hard sometimes. The local tranny guy was going to work on it, but said he couldn't get info from GM to do repairs; said it was OK to drive for awhile. Took it to the deale...