While a Minecraft world can technically be up to 8 times the surface area of the Earth, nobody actually has a world that big, you'd run out of disk space before even getting close.
In general, I've only explored around 2-4km from my spawn in each direction in my SP world. On Robert Maxwell
's server, which I infrequently (
) play on, people have explored maybe 10-20km out.
There is one guy that's doing a Youtube series where he's attempting to walk all the way to the edge of the map. He has been doing it for about a year now and, extrapolating from the progress he has made in that time, it will probably take him about 20 more to reach it. Here's a map of his progress