It takes about 10 to 15 minutes but the timing will vary depending on how large your chicken thighs are, and how hot the grill is. If your grill is hot, 350 to 400 degrees with the lid down, it will take about 12 minutes (or 5 to 6 minutes per side) on average. The key is to use a digital thermometer so you do not have to guess if your chicken thighs are done.