I'm not positive about this, but will assume the game starts you in the center of the map. Using the largest option available on Switch, the distance to an edge traveling straight in an X or Y direction would be 1536 blocks (more going diagonally to a corner). A block is supposed to be a cubic meter. Google says normal walking speed in Minecraft is 4.317 meters/second. So if it was completely flat land, walking at regular speed from the center to an edge would take about 6 minutes 56 seconds. For the smaller worlds of X360/PS3/WiiU, it would be about 1 minute 40 seconds.
EDIT: Speaking of differing world sizes... I know this game will eventually support world import from Wii U version, and similar things are possible for the other consoles across generations. Do the worlds always come across as the same size? I don't even have the Wii U version, I'm just curious if a player could import their Wii U world but have its entirety surrounded by a lot of newly generated land.