Time is the critical factor. Under the
Fair Credit Billing Act Act (12 C.F.R. § 1026.12). With ATM or debit cards, you must act quickly to avoid full liability for unauthorized charges when your card is lost or stolen. Under the federal Electronic Fund Transfer Act, your liability is:
$0 if you report the loss or theft of the card immediately and before any unauthorized charges are made.
up to $50 if you notify the bank within two business days after you realize the card is missing
up to $500 if you fail to notify the bank within two business days after you realize the card is missing, but do notify the bank within 60 days after your bank statement is mailed to you listing the unauthorized withdrawals, or
unlimited if you fail to notify the bank within 60 days after your bank statement is mailed to you listing the unauthorized withdrawals. (15 U.S. Code § 1693g).
If you can convince the bank that your notification failure was due to extenuating circumstances, it must extend the notification timeline for a "reasonable period."
Note that if the theft involved only your ATM or debit card number—and not the card—you're not liable for unauthorized transactions if you report them within 60 days after your statement is sent to you. It's important to review your statements.
I work with three different banks for different commercial reasons, and all three have an 'alert over x amount' feature. They're worth their weight in gold.