Hey Ed, timing depends on the size of the racks. I don't do baby backs much (I prefer spares), but I think the timing is pretty similar. I find about 5 1/2 - 5 3/4 hours on spares. Water pan in the bottom (aluminum mini-loaf pan fits well), 2.5 oz of wood, and let 'er rip. I don't even peek until 5 hours. Some put sauce and/or spritz with apple juice in the last hour or two, but I don't even do that - and the ribs are still killer.
As far as timing, you might want to start doing the toothpick test at around 5 hours and see how they look!