I realize there are a number of difficulties posed in this question, related to Time, Homer's sense of agelessness through the series, etc...But lets say he's in his mid-to-late 30s, and that hes been something of a Doughnut Fiend since he was like 6 or 7. That would give him 30 years or so of snarfing down doughnuts.
How many doughnuts does he eat on a given day? Well he seems to put them away regularly during his breaks at the power plant--I'd say at least half a dozen there, every day, 5 or 6 days a week. I have a hunch hes also the kind of guy who can't just DRIVE PAST Krispy Kreme or Dunkin Doughnuts--say another half dozen there. We'll be kind and say Homer rests from Doughnut consumption on the 7th Day, Sunday. Thats still in the neighborhood of 70 doughnuts a week.
70 x 52 weeks = 3640 doughnuts a year 3640 x 30 years =109,200 doughnuts in his lifetime.
If you were to take all those doughnuts and roll them up into one big doughnut, it would probably be about 12-13 feet wide and 4-5 feet tall. The hole in the middle would be large enough to fit even a fairly portly human being, and with the help of alien technology taking advantage of the chemical makeup of the doughnut, it would probably be comfy enough to sustain Homer on a long spaceflight in his new AstroDoughnut.