How many days an overdue credit card is considered serious is a relative issue, but generally speaking, an overdue credit card of more than 30 days is considered serious. The following are detailed instructions:

1. Overdue for less than 30 days: Overdue for less than 30 days is generally considered a minor breach of contract and may only result in some additional fees or changes in interest calculations. At this time, overdue amounts can be paid off promptly to avoid further repercussions.

2. Overdue for more than 30 days: Overdue for more than 30 days may be considered a more serious situation. At this time, banks often take some measures, such as reporting to credit bureaus, stopping providing credit lines, etc. This can have a long-term negative impact on a person's credit history and make it more difficult to apply for a loan or credit card in the future.

3. Further consequences: If the overdue period continues to be extended, there may be more serious consequences. The bank may take legal action to recover the amount owed and take the matter to court. Not only does this increase the cost of litigation and the complexity of legal disputes, it can also cause greater damage to an individual's credibility.

Summary: The number of days an overdue credit card is considered serious mainly depends on the length of the overdue period. Generally speaking, being overdue for more than 30 days is considered serious and will have a long-term negative impact on one's credit record. Therefore, timely repayment of overdue amounts is crucial.

Extended information: In China, banks and credit card institutions will formulate specific overdue management regulations in accordance with national laws and regulatory policies. These provisions usually include provisions on overdue interest, penalty amounts and consequences. Therefore, it is recommended that cardholders carefully read the relevant terms when applying for a credit card and abide by the contract to avoid overdue problems.