Originally Posted by pandelion
Question: is the function of topping off with more juice in the secondary simply to fill headspace?.
A: It's just to fill headspace.
As for keeping it in primary for bulk aging, I wouldn't keep it in primary for more than 3 months, even 2 is pushing it. You get some strange flavors if you let it sit on the yeast for that long.
Primary:Russian River Redemption clone, Kelly's Melomel, Graham's English Cider 22-23
Aging:Public House Dry Stout, Procrastination Porter, Mr. Brown Ale, Westvleteren 12 Clone, Mead, Duvel Clone, Graham's English Cider 6-21, Belgian Draak Strong Ale, Fig Melomel, Acerglyn, Restorative Tonic Metheglyn