I read up on this a couple of months back. It will last 6 months. It is basically an advance of half of your child tax credit. So if you receive a child tax credit and still owe the IRS, anon would advise to save the money rather than spend it.
"By July 1, the IRS will open a portal that's designed specifically for the new child tax credit payments. You can also use it to opt out of receiving multiple payments this year."
https://www.cnet.com/personal-finance/how-to-opt-out-of-the-child-tax-credit-advance-payments/