The correct answer is as long as it takes :)
It depends on the temp you re cooking at and how big the racks are. here is what works for me, but YMMV...
Baby Backs (2.5 - 3 lbs per rack) - 270-ish, 2.5-1.5-glaze
St. Louis trimmed spares - 270-ish, 2.5-1.5-30-glaze
But, every rack is different, so those are approximate. Use either a toothpick between the bones or the bend test for see when they are done. The toothpick should slide in with no resistance. For the bend test if you pick up a rack from one end it should bend easily to about 90 degrees and the bark will start to crack. There is a great picture around here somewhere of that. I think Northwestbbq posted it.
BTW, it looks like you didn't trim enough to be St. Louis trimmed. The best way is to get the rack straight and find the longest bon and trim straight across at the top of that bone.
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