According to the Consumer Financial Protection Bureau:
Report your lost or stolen card or PIN within two business days of when you discover it is missing, so you limit your losses to $50 or less, no matter how much is charged to your card.
Let’s say you lost your debit card or it was stolen. If you notify your bank or credit union within two business days of discovering the loss or theft of the card, the bank or credit union can’t hold you responsible for more than the amount of any unauthorized transactions or $50, whichever is less. If you notify your bank or credit union after two business days, you could be responsible for up to $500 in unauthorized transactions.
Also, if your bank or credit union sends your statement that shows an unauthorized withdrawal, you should notify them within 60 days. If you wait longer, you could also have to pay the full amount of any transactions that occurred after the 60-day period and before you notify your bank or credit union. To hold you responsible for those transactions, your bank or credit union has to show that if you notified them before the end of the 60-day period, the transactions would not have occurred.