According to the in-game tooltip, the official site, and practically every other source the duration of the Viking's transform ability is 3 seconds (either from ground to air or air to ground). However, when transforming a large group of vikings it seems that they don't all finish the transformation at precisely the same instant.
I've read that this divergence is simply created by them moving a little before starting the transformation, for spacing, but upon testing it behaves the same even if I space them in advance. Besides, it happens in the ground to air transformation as well, which doesn't really have any positioning issues.
Is it just a cosmetic effect and the ability does have a constant duration, or does it actually have a random element, just like burrowing has a random element in the duration?