The initial fireball of a nuke is caused by xrays absorbed by air and develops in microseconds. Basically all at once, since the xrays move at the speed of light. A spherical region of air just ionizes.
The expanding fireball moves quite a bit slower and increases 10s of meters (from an initial ~10-100 m) over several milliseconds[1]. After that it slows down even more, and if you measure every millisecond you'll see the fireball expanding by meters.