Togster,
First of all congrats on the new cooker.
I cook in an 009 and I will usually do 2-3 butts at a time. I like to take the internal temp to 200-205 degrees to get an easy pull.
Through trial and error(mostly error)I always plan on at least 18+ hours. I have not had a bad butt yet(no pun intended).
Keep at it...the fun has just begun.