Youth football - coach speak

[DLH17]

"How far behind the line of scrimmage can their (running) back be (prior to the snap)?"

He was stressed because the opposing offense was in a "hurry up" mode so he was fishing for anything to distract the officials in an effort to slow the game down at that moment.

I don't know that I've ever read a rule or case play that addresses formations specifically related to how far off/max distance (behind) the LOS a back can line up.

Didn't find anything in the online rules using key word search function.

Thoughts?