How long had the cannibals been able to get into the tunnels and eat outsiders?

OK, the cave-in was in 1892 and the movie, we will assume, is set in 1971 or 72. An 80 year span. How long did the survivors of the cave-in have to live only on rats and their own dead until one of them made their way into the tunnels and found other people? Was "The Man" the first one to get out?

I figure the lifespan of the cannibals was pretty short. They would probably begin to reproduce at puberty and die in their 30s or early 40s due to their unhealthy environment. Birth rate and child survival was probably extremely low. I believe it is stated in the film that the original group were 8 men and 4 women, likely in their 20s or 30s. As manual laborers of the 1890s, they would likely have been mostly uneducated people with no way of keeping civilized traditions going.

1892-1893: first generation of cannibals born - survivors try to hold together as best as possible.
1905-1906: second generation - society becomes more and more savage by the year.
1918-1919: third generation - all original survivors probably dead at this point. Diet mainly rats, bugs.
1931-1932: fourth generation - total breakdown of language & social structure. Declining birth rate.
1944-1945: fifth generation - "The Man's" generation. All memory of outside lost by now. Population shrunk to a mere handful.
1957-1958: sixth generation - no children survived except possibly "The Woman".
1970-1971: seventh generation - Population is down to two members. Generation is unborn due to the death in pregnancy of "The Woman".

They must have breached the cave-in no earlier than the fourth or fifth generation when all memories of the outside world were lost, as there was no attempt to make civilized contact and seek rescue. My opinion is that The Man was the first to make it out in his desperation to find better food for his mate. I don't think the disappearances had been going on more than a few months or years at most, although this raises the question of whether or not they could have survived on rat meat and insects and the occasional death of one of their own. Probably at first, The Man was just killing vagrants and did not attract the attention of authorities until he killed the upper-class "gentleman" on his way home from the porno shows.