rule of thumb is 30,000 miles but check your car owners book, my buick says 60,000 miles, i didnt do it till 80,000 and the auto trans started to jerk shift at low speeds like it had a shift kit and i looked up on line where people were being sold solenoids by shops for this problem which come with a fluid change and about $480 repair, all i did was go thru a fast lube place where you sit in your car while they change fluid for $84 and filter and 200 mile down the road it was shifting smooth again like new, the new fliter flush the new oil thru the valve body and clean it of being gummed up a little. Stay clear of a line flush unless they do the flush by removing the pan and flushing from the pump and replacing the filter. A line flush is not good and its a cheap way to make money with little labor, it leave you with an old wore out filter in and maybe metal or dirt chunks in the pan that wont go thru line and are stuck to the magnet that could pop loose on bumps etc
I would recommend changing it every 30,000 because trust me Ford wants you to have to repair it once the 60k mi powertrain warranty is over. be thankful its not a GM or Chrysler trans, those need to be flushed every 15k. GM automatics are the most fragile
The poster of Answer #2 has absolutely no idea what they're talking about. Most GM transmissions are designed to go 100K miles before a fluid change under normal conditions, 50K under severe service and are far sturdier than Ford or Chrsyler.