By the time announcements like this are made, there have already been lengthy successful IT pilots with much change management planning already conducted, partners engaged, etc. If they say 12-18mo, they may slip to 18-24mo, but definitely not multiple years.

That said, the other commenter is also accurate: a "full" roll-out will not mean 100% replacement. Things would break and it would be an irresponsible management decision to force some things that shouldn't be forced.

Source: I led a large MSO-->G Suite migration in a previous role.