X-Men Prime, which introduced the changes from Age of Apocalypse, showed us adult Marrow bumping into Dark Beast in the Morlock tunnels. She called him "First One," and he told her how he genetically created the Morlocks "a generation or two" ago and how most of them were big disappointments to him.

Sugar Man arrived in the 616 twenty years in the past and significant altered the course of Genosha, turning it into what we already knew. The twenty years sort of made a bit of sense, considering the focal point of AOA was Xavier dying 20 years in the past.

Pretty sure Dark Beast's vague "generation or two" was 20 years, as well.