I'm about 40% gray at this point and leaving the color on for the recommended 20-25 minutes wasn't really doing much to get through the gray at all (I use Feria).
But what I do now is leave it on for about 40 minutes, and that does the trick with no problem. I'm sure you're really not supposed to leave it on that long but it does work.
But if you're going to do that, definitely experiment with how long to leave it on beyond the 20-25 minutes. Start in 5 minute increments so you don't end up with color that's too dark.