Hi Gary,
I'm with you on 8-12 hours using a typical ratio of 1/2 cup table salt (or 3/4 cup Morton Kosher or 1 cup Diamond Kosher) to 1 gallon of water.
I've brined a few turkeys longer, and have ended up with almost watery turkey and texture changes that I don't like. But your mileage may vary.
If you have the option, you might consider using a high-salt brine, but for a shorter time. For example, use a ratio of 1 cup of table salt (or 1-1/2 cups Morton Kosher or 2 cups Diamond Kosher) to 1 gallon of water, and a brine time of only 4-6 hours, followed by a thorough rinse.
Regards,
Chris