I am not a swaminarayan sect vaishnava but a visishtadvaiti, hence can answer it.
The idea in which it is said is a general rule like after missing a human life, a soul suffers in hell or enjoys in heaven for some period, then based on its karma starts its cycle as a living thing on earth. By sticking to its karma, it upgrades and becomes human.
There are more possibility that many years are consumed in this transit, however specific number seems to be unwarranted by srutis, but as a statement of a guru, for that sect it holds validity
Jai shree krishna