Electronics - we keep the boxes forever, in case we move/need repair etc. Those perfectly molded styrofoam pieces are too useful to get rid of. We’ve got an enormous attic though.
Everything else - as soon as the 30 or 90 day return period is over, the box gets put in the basement, where it sits some indeterminate amount of time until I get around to putting it out with the trash.
I have to keep an eye on my wife. Her dad had the original box for everything he ever bought, 40 years later. I can see keeping computer cartons, but not for the coffeemaker or the toaster.
I don’t keep many boxes. I do have the ones for my stereo and speakers, which I bought in '98. It’s pretty good stuff, and I figured the boxes would be handy when moving. Which has happened one time, since 1998. 
Other than that, nothing really. I even tossed the box for our new 42" plasma TV we bought last year. Mainly on account of it being so fricking HUGE. If it breaks down, they can take it back without the box, dammit. THEY make the boxes, not me.
As someone who, just last week, ended up with 7 large boxes that contained brand new, electric space heaters that I had to purchase (due to a broken water line, sub-zero temperatures, capped drainage line that goes out of our basement into the city ditch, and 3 feet of standing water in the basement that totally fried a nearly new, 4 month old furnace -oddly, the 40+ year old Philco fridge that sits next to the furnace is STILL running…that sort of creeped me out), I tossed them all.
I had no room for them. Just too much clutter, in an already cluttered, stressful situation!
All the heaters work just fine, and since we got our new, replacement furnace installed today, I’m thinking of giving all but a couple of the heaters to charity…after all, insurance is going to reimburse me for them (Yes, I kept the receipts), so I might as well pass them along to someone who might need them, and can’t afford them…sans boxes, of course!
I keep 'em for a long long time. The styrofoam comes out and gets strategically located around the apartment, the boxes get flattened and placed elsewhere. Small appliances (humidifier, etc…) stay in the box when not needed, because it’s easier to stack boxes than appliances on each other.
This has mostly because I’ve been a tenant with cross-country moves over the past decade, so keeping the boxes makes sense. Once I move into my own place this summer, I will ditch all the big boxes because stuff won’t be moving anymore!