Alison accidentally omitted $40,000 of gross income from the restaurant she owned on her 2015 tax return.

Question:

Alison accidentally omitted $40,000 of gross income from the restaurant she owned on her 2015 tax return. The return showed gross income of $150,000 when filed on October 15, 2016. When can the IRS no longer pursue Alison with the threat of collection of the related tax, interest, and penalties (assuming there was no fraud)?