We never really did that. We have waited after all of the top allergen foods like gluten, dairy, eggs, soy, peanut butter, fish, etc but that's it. If he started having reactions we were fine with doing diet elimination and if he was reacting to different
We did for the basic fruits and vegetables in the beginning but now we don't at all. Once we introduce high allergin foods like PP then I would probably do 3 days again.
Re: New food
No. If it was a high allergy food we would. DH and I do not have any allergies and like you said it would take forever.
In the beginning we only did 2 days. My pedi said if there was an allergy it would have been seen immediately.