I learnt for one year. Then I picked up piano again when I was 35. I have been learning it for one year. I passed grade 3 with honor already. My experience is dont start too early. As an adult I grasp all the theory very well. I dont really need to memorize them as I can figure out all major, minor keys, tiras chords acurately based on the theory.
I would say start learning when the kid is 10+ years old. So he is old enough to understand the theory. From there he can improve a lot faster. Younger kids started hate piano if you push too hard. Older kids can regulate themselves.
If the kid started at 4 or 5 it will take the kid 3-4 years to pass grade 3. If the kid started a little later 10+ or an adult with some musical sense there is no problem to pass grade 3 in one year. Consider 3-4 years effort vs one year.
I travel a lot with my job so I can not practice every day but if I have time I practice for at least 2 hours a day.