Ball joint replacement on 1997 Ford Explorer

How often do ball joints need to be replaced and how can I tell if they're going bad? I have had all four replaced in the last 5 yrs and now am told by one garage that one has 1/4 in. left and needs replacing now, while another garage says the lower ones "have too much play" and need replacing again! Who do I believe?

1 answer
If the ball joints need replacement again make sure they use the best quality ball joints available ( Moog or McQuay-Norris ) as the cheaper ball joints save you money but you end up replacing them sooner