I got through one of the second largest as a kid.
Took me a few months, but it went faster once I hit the center. It's filled with a gumball, by the way.
But once you get there, it looks like a cut out of the earth with all the rings.
I just had to start licking in circles for hours and hours, slowly working my way out to larger circles, jamming my tongue in there to cover more area.