L4D - get as many people with guns together as possible together. We're probably last... 2 weeks... if that. Also head to the bloody coast. <== key. Always made me wonder why they didn't stop at a major city to get into the harbor... plenty of boats would be there, or even a small coastal town.
If it's not L4D style and your normal zombies... I give humans an 50/50 chance of surviving. It really depends upon food consumption above all else. Plenty of zombies will be killed in the cross fire of humans (at least in America) though my only real concern is the food supply. Arming everyone is fairly easy and at this type of epidemic level, any-person with a shit load of guns is going to be giving every person they meet a gun in order to fight off the hordes.
Though realistically - about 60% of the people who think they would survive it won't.