I can use the commercial cedar planks several times before they are finished (then just use the scraps as smoke wood). I use my OTG grill, grill direct over medium heat (in my 22.5" OTG I dump one chimney of lit and spread it evenly over the charcoal grate). I preheat the soaked plank two-three minutes (by the clock) on the fish side, then flip the plank and let it heat until it starts smoking. I then put the fish on. It'll take 15-20 minutes until the fish can be flaked then it is done. Some compound butter (a pat per piece) with the heat of the filet melting it makes a great finishing sauce.
My son goes to the lumber yard (or Home Depot or Lowes) buys an untreated cedar plank and cuts it into several lengths. These planks are MUCH thicker and will last much longer (can do at least twice as many times). These planks are nominally 1" thick (really about 3/4") whereas the commercial grilling planks are only about 3/8" thick. Plus, the lumber mill boards are often MUCH less expensive. You do have to have a saw, tho' (or have the lumberyard cut them to length).
If you wait until after the grilling season starts, Costco puts the remainders on sale and you can buy a package of planks for about a third of the price. That's when I stock up.
Keep on smokin',
Dale53