How long before you receive your credit card through the mail if send it off on a Monday?

Generally, the United States Post Office (USPS) promises delivery within 2 and 3 days for all mail sent between addresses within the Continental US. The USPS adds 1 day for mail sent to or from Hawaii and Alaska.

If the credit card company told you that they sent the new/replacement card on a Monday, there is a high likelihood that the mail did not actually get into the postal system until Tuesday morning (as credit card companies tend to batch all mail prep at night).

Assuming that there are no legal holidays during the week that members of the USPS will take off, one would receive their new credit card on Thursday or Friday.