Tiny acreage accidentally omitted from a gas unit?

I own a very small (1 acre) mineral interest in east Texas that I felt like had been inadvertently omitted from a gas unit (created in 2006). I contacted the operator a few months ago, they agreed, and promptly put me in pay status.

My question is for how far back should they pay me in missed royalty payments?

Hard to say without knowing all of the details, but you will probably be owed all the way back to when the unit was formed.