For the world record, as for the other attempts, I personally started as soon as the loading screen finished and the actual game animation (where Ralph walks up to the building) starts, and I end it as soon as the "YOU FIXED IT!" screen comes. The timing aspect for when to start became a bit more of intuition as soon as the the loading screen jingle reached the 2nd note on its 2nd cycle (you'll get this the more you play it)
But as for the timing, refer to what I've said above.

Great, that’s the timing method I’ve been using