Actually the current answer is wrong.
If you wait longer you will have a turbo that last longer (not stronger). The more you do them the longer it will stay once you stop doing any.
You can just try it in an arena, do a 3 times very short turbo twice and go in straight line without doing anything. Repeat with doing longer, it will last longer.
This is ...
I'm pretty sure the boost is at its maximum potential the moment the power bar turns red. And even if there does happen to be a slight increase in speed, it isn't significant enough to risk the many (of which some will be inevitable) backfiring boosts.