Chris,
Welcome to the forum and the great world of cooking on the WSM!
I just did a flat about that size last weekend at 250 degrees. I can't recall how long it took - sorry. I use a thermometer in the meat and with briskets I start checking for done-ness when it hits 180 or so. There are lots of threads and articles on briskets on this site to gain knowledge from. Be patient; brisket is done when it's done, and before it's done it ain't so great. I'd also caution you not get too carried away with your rub on a smaller flat like yours. It's a relatively thin piece of meat, and if your rub is really bold it'll overwhelm the good brisket flavor. I foil at about 175 degrees and save the au jus to serve with the brisket.
If it doesn't thrill you when it's done, you can do what I did with my first few briskets: cube it up and stew it into some chili. It's fabu-licious.
Dave