In most states, they are allowed to repossess as soon as the payment is late. It's more typical for them to wait between 30 and 90 days, depending on past payment history. There's a better chance of more time if this is the first late payment. (If at all possible, it's much better to try to refinance or negotiate a catch-up schedule with the lender before it happens.)
Does anyone have personal knowledge of how many months of non-payment for a car to be repo'd? (I'm helping a friend gather info)
It depends on what, if anything, the loan agreement states.
If it's not covered in the loan agreement, it may be stated in the state laws. The problem comes when the loan agreement says it will be administered under StateA laws, when both (or one) party is in StateB.
My niece's car was repossessed after she missed one payment. She had her debit card hacked, and when she opened a new bank account, they forgot to tell she would need to re-establish all her automatic payments.