If a largesum of money was wired to my bank account but I know nothing about it, when is it mine to legally use?

If you are not expecting this money, and it is no way shape or form your money, then it is actually considered in many states to be a crime to keep said monies. Further, if this is due to a scam of some sort, wherein someone is either attempting to gain access to your account or what has become prevalent in many situations, where you cash a check and your check doesn't clear, you could become liable for a lot more than the original amount deposited. The availability of monies is different than if the money is actually "good".  Under the Expedited Funds Availability Act, banks must make a certain sum available at certain time periods. However, even if the bank makes all of the funds available to you within that month, and you spend it on the 31st day or anytime, the bank could legally force you to pay that money back if the check is returned as insufficient, a scam, a fake or as a simple mistake. You are responsible based on federal and most state law to know what you deposit and what you withdraw. As to these funds, you should contact your bank, find out how much and from whom, and go through the process to see if the bank could research to see if the funds are good. If you can get this in writing, that the funds are good, you may have more hope in keeping the money.