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!