I've only found a few that had enough meat to slow cook. I just went by texture. I wanted the fat rendered down and the meat to be pull apart tender. I was think'n they took a good 5-6hrs, similar to spare ribs. But it has been awhile. Take them off too soon and the beef ribs are tough, btdt.
8hrs for spare ribs at 225? I'm guess'n they better be baste'n or they will be bone dry. I like to do my ribs at 235 and they tend to take 5-6 hrs. But I'm talk'n smoker and not conv. oven. So take it for what it's worth.
Plan on 6-8hrs, but go by feel. Good luck!
BTW, welcome to DC.