I use age-linked media as my proxy for attention span expectations:
-The average toddler book takes about 2 minutes to read without dialogic reading.
-The vignettes on one the two children's TV show I watch with DS (Between the Lions) are only a minute or two long.
-Our music classes have segments grouped into about 2-3 minutes each.

DS20mo has no trouble focusing on an activity for half an hour or more at a time. At the extreme upper end, he's asked to read one book after another continuously for 4 hours on more than one occasion.

What is to give light must endure burning.