Most Zombie movies portray an outbreak as an overnight occurance, but in reality this most likely will not be the case. Based on rate of infection and incubation times, you could have anywhere from a few days to weeks to prepare for the deluge of the undead.
To determine how fast the infection could spread, lets look at the last immense biological threat we faced, H1N1 or “Swine Flu”. Generally all types of influenza have the same incubation period, which is 2-7 days. The times vary based on the inital health of the patient and their metabolism at the time of infection. Assuming the shortest incubation period, and that a zombie will infect only 10 others in a 2 day period, lets look at the time table for the infection to spread through out the entire United States.
Day 1: 1 zombie
Day 3: 11 zombies
Day 5: 110 zombies (By now you may have heard some news stories, but they could be unconfirmed.)
Day 7: 1,100 zombies (You will have heard about the outbreak on the national news by now, and the infection will have spread coast to coast.)
Day 9: 11,000 zombies (More news stories. Smart people will have begun stockpiling food and other resources)
Day 11: 110,000 zombies (1 in 3,000 people are now infected. The Government will become involved.)
Day 13: 1,100,000 zombies (1 in 300 people are now infected. Odds are, someone you know is infected. Government recalls all deployed troops.)
Day 15: 11,000,000 zombies (1 in 30 people are now infected. Someone in your family is probably infected. Government declares Martial Law.)
Day 17: 110,000,000 zombies (1 in 3 people are now infected. Odds are, you’ve come in direct contact with zombies, or are buttoned up in your house.)
Day 19: 1.1 Billion zombies (Entire United States has been over run and the infection has spread to the rest of the world.)
Day 21: The entire planet has been consumed by the undead except the lucky few, and only the most remote locations remain untouched.
With the infection spreading in this fashion, could you possibly survive? Well, suprisingly, your odds are not too slim. If you can remain inside and safe until day 21, then your odds of survival increase dramatically. With almost every human on earth infected, the zombies are completely out of food unless they go after animals. Assuming they don’t, they will begin to starve, and if you’re really lucky, they’ll begin to dehydrate. If they dehydrate, then you will only have to wait about another week before its probably safe to go out and rebuild society, but if they have to starve, then it could be another month before you can go out to scavenge and rebuild. This is why it is important to have a supply of food and water to help you stay inside, and stay safe. For this you can make a Button Up Barrel. Good luck!