Like clockwork, right at 3k miles. I was on my final leg of a long round trip and had the issue begin during very hot riding through the desert here in Arizona (105°+). The hotter the tank gets in direct sunlight, the more that stuff seems to loosen up. In the end, I guess that helps with the removal of it.
For those who have tanks that haven't really gotten that hot, it may be an idea to set it out in direct sunlight for a few hours and then do the tank rinses. Well, in my climate that would work right now but for the lucky ones that aren't dealing with our heat maybe there's another way to heat up the tank without damaging it.