Ok...at 225*-250* cooker temperature.. figure on 1.5 hours (+or-) per pound for a full size (8-10lb) butt! For partial sizes (2-3-4 lbs) I don't have a clue.
To the meat..None... To you...a lot. How well you maintain a constant temperature (Fire Control) is the key. The weather affects your ability to maintain that constant temperature. You will find it more of a challenge when it's 35*, North wind at 15 MPH, and misting rain than when it's 80* Calm winds, and sunshine...HTH
PS...When it's windy move your cooker out of the wind.
I'd say under perfect conditions 4-6 hours on a chimney full of briquettes is stretching it a little. I went through a large bag and a half for 10-12 hours keeping the cooking temp at 225.
Like UB said, there are plenty of factors...even the design or materials of your smoker/cooker has an affect.