When buried six feet down, without a coffin, in ordinary soil, an unembalmed adult normally takes eight to twelve years to decompose to a skeleton. However if placed in a coffin the body can take many years longer, depending on type of wood used. For example a solid oak coffin will hughly slow down the process.
Assuming everyone is buried without a coffin and in normal soil, there would be 31517016 zombies purely risen from the dead. You would be wondering why 31517016 zombies, the math is, 2626418 is the average number of people dieing per annum and 12 is the number for years (maximum) for a body to decompose, 2626418 × 12 = 31517016 All the dead people buried before 12 years ago, would have already decompose and will not be considered undead. So with 7 Billion living humans (with modern technology) verses 31517016 undead classic zombies, who as slow, witless and only react to sound, We can survive a zombie apocalypse with ease.
BUT
If the undead is considered to be all the dead people till now, that would be 107 billion zombie against 7 billion zombie. Making that around 15 zombies for every 1 living human. The chances of surviving is slim.