Basically, if you watch the intro all the way, timing can start in one of two ways.

-The instant you press the Murfy button to slap the heroes -The instant you press pause to exit level

Basically the first input that the game lets you do.

If you skipped the intro cutscene, timing begins as soon as Rayman spawns into the center of the Main Gallery. Doesn't matter if you have already entered a level, so long as you have 0 teensies/haven't completed any level.

It's about 1 minute 32 seconds from selecting a new game to first input