Three months then they can take action against you either taking your car or garnish your wages or take you to court. most likely take your car.
If you are having problems making your monthly payments it is always better to contact your lending institution to see what can be done. Usually the only thing that is done is that your vehicle is reposed and sold at auction. You will have a certain amount of time to pay before this happens. It would be highly unusual for the lending institution to go to court to garnish your wages. Also the lending institution is not in the car business so they really don't want your vehicle they only want your money.
On the average, they will usually let you go 2 months, but depending no the car lot, I have had friends who if they go behind one week they would come get the car. But in general big finance companies will usually work with you so long as you avoid them. Let them know what is going on and they should work with you.