“Share prices of breached companies hit a low point approximately 14 market days following a breach. Share prices fall 2.89% on average, and underperform the NASDAQ by -4.6%,” according to this study:
Analysis: How data breaches affect stock market share prices (2018 update) – Comparitech
You didn’t ask about the longer term, but I found this quote insightful:
“In the longer term, share prices continue to grow, but not fast enough to keep up with the NASDAQ. After one year, share price has grown 8.53% on average, but underperforms the NASDAQ by -3.7%.”