So i ordered a set of d'addario exp 13-56 wich are coated. I dont play my acoustic that often, and wondered how often i shuld change them. I've heard a guy say that they last for years. Is that true?
No strings will last for years. If you're playing regularly (every day) you should be changing them around once a month, or once every two months if you're playing less. Regular wear and tear from playing will break down the coating on the strings.
Ya the coating is supposed to help them last longer and it does for sure, but I'd say the best way to prevent strings from getting tarnished and sounded flat is to clean them after playing and keep the guitar in a case. If you play only once or twice a month and wipe down the strings after playing and keep the guitar cased up I would leave them on for a year sure.

I have 6 acoustics and one of them is a thinline A/E that I only ever use for recording leads, which means I maybe play it once or twice a year. I last restrung it in February 2010 with some regular Martin strings and pulled it out on Friday and they still sound great.

that being said, I have an Epi masterbilt dread that I play nearly every single day for about an hour or so and I leave it on a stand in the living room. That sucker needs new strings every 2-3 weeks.
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I say once a month if you play it a lot, and once every 3-6 months if you rarely pick it up. If you never play it then strings will maintain their brightness and tone, but you have to keep it in a case, if you just leave it out, the strings will loose their tone.

Make sure you clean your strings after you play, even coated strings get crud built up on them and it can eat away at the coating.
You change the strings when you don't like what it sounds like.

It's that simple.
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i like what old strings sound like but i change them anyway because I like what new strings sound like better
changing strings completely depends on how often you play coated strings last a little longer than uncoated ones, so change them when they appear dirty or the sound starts to get shitty
Change your strings every 6 months as a minimum. If you play a lot then every few weeks would be more appropriate.