I've recently heard of a magazine I won't name, which has held on to stories for 8 or 10 months before responding (rejections, but with a 'we like your style, please try us again' note attached).
I have to say, I think six months is the longest we should have to wait for a response. Many writers would give up after no response for 8-10 months and would assume the story was lost or that they'd missed logging the rejection. You wouldn't blame them for subbing the story elsewhere.
Come on mag editors. We try our hardest to please you at all times - in return, please try to respond within six months. Pretty pleeeease!