ADP/ACP corrections

recently took over a non-safe harbor 401(k).

Both ADP and ACP tests for the last several years have failed.

Is there a specific time frame to correct?

See the current EPCRS rev proc, I believe it is covered there. 

Generally speaking for a calendar year plan if my memory servs its

By following 3/15 - no penalties.

By following 12/31 - 10% excise tax and Form 5330 but no other corrections

After 12/31 - refunds, 5330(s) and 1:1 QNEC correction.

I think if it is corrected in the 2 year window following failure you may now be able to self-correct but if it's more than 2 years you may need VCP but again I think those time standards are in the EPCRS procedure and should be relied on more than my memory.