The crock pot method works well, and especially with the apple juice in the bottom for moisture. I would keep them on low for at least 3 hours, but you may need more time depending on warm you want them. The ribs need to be completely thawed out before you put them in the crock pot, or that thawing will add more time! I did a big crock pot of pulled pork a few weeks ago, with apple juice, for a birthday party. I think the pork was fully warmed around 3 hours in (on low).