Spare ribs take longer than that on average. Depends on cooking temp but 12 hours is way too long.
At 250F check for bendyness at 3.5 hours. Im not saying it is done, I am saying check and make decisions. Around that time I will probably wrap them for half an hour. Then glaze and crank the temp higher until it sets.
Fyi i am checking
-Bend test. Pick up the rack with tongs 1/3 of the way through. If it bends more than 45 degrees its prob done. More than that it is over. Less and it is not cooked enough
-Bark. What is the color? Dark? Too dark? Is it hard, soft etc. Sometimes you need to wrap your ribs with some butter and honey, sometimes you dont. Its a game time decision. I prefer parchment paper or butcher over foil. Foil gets too steamy.
Pro tip: if you see the bark crack when you pick it up thats a good sign
Start to finish id say 5-5.5 hours for ribs. Depends how meaty, fire temp etc