You can stay as long as your form is valid. However, you may not "enter" the US. Note that "entering" and "staying" are veery different. The Visa (stamp in your passport) is for entering, the "status" (in your form) is for staying.
My visa expired last year so I went to Tijuana to get a new one. It was very easy and straightforward. I went to the embassy in the morning and had my passport and visa back by 3 pm. However, check with the American Embassy website, and particularly the Tijuana consulate. They might not be able to or may take longer to process visa applications depending on which couontry you are from. Good luck
