It really depends on the lender.
Generally, two or three consecutive missed payments can lead to potential repossession, not to mention that it can damage your credit score tremendously. Some lenders will even consider repossession after one missed payment.
The good news: You usually have options to handle a missed payment before things get out of hand. Be upfront with your lender about your circumstances and try to work out a solution.
Some ways to help lessen the burden of your car loan include:
Asking for an extension or alternate payment plan
Adding additional months to the loan
MORE: How to refinance your car loan