I never said they will certainly attain enlightenment by
practicing for a fixed amount of time. Some may take months, some
take years, some take decades and may not be enlightened before
death. (but that's ok as Buddhist scripture accepts that for some
people it will take lifetimes of practice before
enlightenment)
It is an eventuality that everyone will become enlightened if they
practice and learn the dharma, but each person has individual
capacities, ability, talent, etc, much like learning piano varies
for person to person. A prodigy may take 1/10 of the time a normal
person takes to master piano, while those who are slower may take
longer time. Same goes to practitioners, each has different
capacities. Some people become enlightened by listening to a verse
of dharma, some people take decades of practice.
But I said, they will definitely see results if they practice --
even in the beginning phases of their practice.