My benchmark temp for just about everything non-poultry is 250*. And at that temp, I found 6 hours (w/o foil) pretty good for rendering out most of the fat-- some small pockets might remain. With foil, you can assure yourself of never having greasy ribs, but you have to get a time/temp combo down that doesn't achieve that at the expense of making the meat mushy. At 250*, 3 hours, followed by just an hour in foil, then finished out of foil for about another hour seems to work for me. It's definitely individual preference, though-- some people cook spares only 4-5 hours, some go 8-9. Good thing is, spares are forgiving, and whatever you end up with will probably not be inedible until you find your own preferred zone.