Even if you could dig that deep, you�d be squashed flat or melted before you got there � by the searing temperatures and crushing pressures inside the planet. The earth, even after all these years, is still cooling down from the outside (crusty bit) first to hot fiery melty bit inside (regardless of global warming, its happening).
We, and everything else on the Earth, live on that thin shell or crust. This is less than six miles thick in some places (like under the deepest ocean trenches), but up to thirty miles thick in others (like under Mount Everest).
Beneath the crust is a layer of semi-molten rock called the mantle. This is about 1,600 miles thick, and makes up about two thirds of the Earth�s entire mass. It�s mostly made of metals like iron, aluminium, magnesium and silicon, all held in a gloopy, plastic state at temperatures of over 1,000�C.
Beneath that, there�s the core of the Earth. This is separated into an inner core and outer core. The liquid outer core is about 1,500 miles thick, and is mostly made of sulphur and iron. This has a temperature of over 3,700�C, and flows round the solid inner core. The inner core is made of iron, and is even hotter (over 4,300�C) than the outer core. But due to a weird effect called pressure freezing, it�s actually solid. So, if you made it down that deep, you�d have a job digging through that!
But let�s say you dig through the solid iron core. You survive the burning temperatures and keep going � through the outer core, the mantle, and the finally the crust � to the other side. You still wouldn�t have made it to New Zealand. Digging straight down from the UK, you�d pop out somewhere south of New Zealand, in the middle of the Antarctic Ocean. Probably quite annoyed with yourself.
As regards how long, depends what you use to dig, a dessert spoon, a shovel or a JCB?