The move has four phases, and only one of them is long. Collecting member emails is quick if the Skool admin gives you every address; if it does not, the pinned opt-in link runs alongside the other phases. Installing Reign, BuddyNext, Learnomy and WB Gamification on staging is a short job, and the free Reign Demo Installer gives you a starting layout.
Rebuilding the classroom is the long phase. Its length depends on how many courses and lessons you recreate in Learnomy and how many videos you move, so rebuild the course members use most first. Then run both sites side by side until members are active on the new one. That period is set by your members, not a calendar, and billing moves last.
If you would rather not do the work yourself, the Wbcom team can run the whole move and give you a timeline once they have seen your courses.