What from? What to?
You cannot plan the migration until you know what you will be migrating from, and what to.
You should refuse to give any estimates until you know the destination platform. If they insist, say "between N weeks and 6 months depending on what the destination platform is".
Meantime if you actually want advice at least tell us what you are migrating from. Presumably you know that much, right?
Biting the hand that feeds IT
