I do quite a bit of plank grilling. I re-use the planks until they fall apart. After the cook, it's a good idea to put the plank (immediately after you take the food off) in a bucket of water (I actually use a large tray full of water) to stop the "fire". Then after the meal is over, the plank gets scraped of all "bits" then washed in hot water. You can actually use soap but do NOT let it sit in soapy water. After rinsing really good, air dry the plank (in the sun is good, if possible) then put in a brown paper sack to keep the plank clean.
I plank on one of my Performers. I use one Weber large full chimney of lit spread equally over the charcoal grate. I put the plank, food side towards the fire, for TWO minutes by the clock. This pre-heats the plank and sterilizes the food side. I flip the plank and let it go until it starts smoking (may take 5-7 minutes) before I put the food on. Then I put the food on, close the lid and start cooking.
Short cooks (like fish) may only take 7.5-20 minutes. If the plank isn't already up to smoke temperature, the fish will be cooked before you get much smoke. At any rate, the shorter cooks will allow you to use the plank several times. The longer ones, like meatloaf, maybe only two. When the plank is "worn out", then break it up for smoke wood.
Keep a spray bottle handy just in case the plank catches fire. Sometimes I have to close off the bottom vent a bit to keep from overheating. The goal is to have the plank smoking during the cook. If it catches fire, that's too much of a good thing. If you do not see smoke throughout the cook, it is not hot enough. Billowing smoke signals that it is too hot.
Keep on smokin',
Dale53