Not to be a smart *ss
But cook 'em until they're done.
Honestly, you cant really go by time. You can guestimate. Say 3.5 hrs minimum, but it may take 4.5- 5...... Ribs are just one of those things.
Smokin' saying of "Its done when its done" holds more true for ribs than anything else.
Not only is each slab of ribs different, but things like the amount of times you open the door, which rack you put them on, and how done do YOU like them dictate the time it takes.
I would at least wait 3.5 hours on the middle rack before I opened the door for the 1st time. But thats just me with my Smokette, in my corner of the world.