When will the victorious skin be revealed?

At or near the end of Worlds.

Victorious Sivir was on the PBE October 29th, which was during the 5.22 PBE cycle.

If we were to guess, It'd probably get announced or teased what role the victorious champion belongs in (marksmen, jg, fighter, etc) around 6.21 and then revealed during the 6.22 cycle for release once the patch comes out. We're in 6.18 starting tomorrow and 6.19 starts on the PBE sometime today.

This is speculation of course.