Are you asking about just a rhizome or a growing plant?

For a rhizome, cover it with about a 1/2" of soil. We've had rhizomes "sprout" just sitting on the potting bench when we've had too many and forgotten to plant them up.

For a growing plant (rhizome + stems), take a look at the base of the stems. You'll see they are a whitish-pink color where they meet the green stem. That's your gauge ... white below ground, green above.

Hope this helps
-Ray