Vertical Smoker Cooking Times
My roommate and I have a Weber Smokey Mountain Cooker, and in general, we've found that smoking times can vary a lot based on a number of different factors that tend to be hard to control. We've done beef brisket about 8 or 10 times now, and we've had some go for 8 hours and some take as long as 14.

It seems to be a function of the amount of connective tissue you need to break down. You've probably already noticed the "smoking plateau" that occurs around the 165-175 degree internal temperature mark -- when that happens there is nothing you can do but wait until the temp starts rising again and hits your target final temperature.

So to answer your question -- for larger pieces of meat, just start early and allow for wide variances in cooking times (we usually start the night before for an afternoon BBQ party). Luckily, most smoked meats taste great 3 or 4 hours after you've pulled them off (if wrapped and stored correctly). Knowing when to put on smaller pieces of meat (e.g. chicken parts/sausages/fish) will just come with experience, but since they rarely take more than an hour or two to cook, you won't be off by that much anyways.