I'm an eternal pessimist. Of all the automatic feeders I've used, the Eheim was the best, after a fair bit of fiddling to convince it not to overfeed. But auto-feeders are like heaters - if they are going to fail, they will always do so when you are far away. I have an acquaintance who feeds every day, all year with an auto feeder. I don't get it, but she likes the set up. The last time she went away for a week, it food dumped her tank.
Two weeks is about the limit for long I'd leave a tank unfed, but they can technically go 3 weeks. I would follow the advice above (to leave the tank unfed) and save my money. You may even save your fish.
I used auto-feeders for school tanks - with Christmas, Easter and March breaks to contend with. In the normal run of life, I avoid them.