Jeff,
I don't have a "set" time, but this past cook, I went 3.5 hours, then wrapped. Most times, I just do 2 racks of spares, but there were 4 this time, and they all didn't "look" ready to be wrapped yet. Looks are the main thing I go for before I wrap. I also want the meat to be tender, but not falling off the bone/mush. I go for falling off the bone on the tips, to having to do a gentle pull at the top where it's thick.