There is no specific ME time for the ATP. Regionals tend to have at 100 ME requirement for insurance purposes. Larger regionals (like Inc) probably self-insure but they still apply similar rules as insurance companies. But if you're willing to accept high risk or insurance premiums, you can hire people with just a ME rating.

Regionals require more than 100 ME only when times are tough and they just want to thin the stack of resumes they have to dig through. Higher ME time usually means part 135 or MEI experience, both of which are more skill-intensive than ASEL CFI work.

It seems like buying 100-ish ME hours and getting on with a large jet regional would pay for itself by the end of the second year.