Like i told you, around 10-11 feet (3-3.5m) but get a flexible measuring tape and measure the length of the path by which you will bring the wire.
You must fuse the batt + to alt wire. You can do this by doing what coldi did, or how blacklist1 did it or simply get a anl fuse holder with 150amp fuse like i did.
About the welding wire, make sure the wire is copper and not copper clad aluminum (even though a 0 gauge CCA wire will also improve the current flow considerably). look at the picture below
