What should the limit on self-determination be?

As can be seen in the link below, self-determination is
generally defined as the right of a national majority (usually defined in ethnic terms)
to choose whether it wishes to rule itself as an independent state, to be part of a
federal system within a larger state, to be fully assimilated into a larger state, or to
occupy some status in between those options.  This was one of the major ideas that
Woodrow Wilson pushed for in his 14 Points that were offered at the Paris Peace
Conference.  Wilson wanted all ethnic groups (or at least white ones) to be able to
choose their own political status.

The question of what the
limits of this should be is a very difficult one.  We see it crop up in places like
Sudan where a region that was formerly part of the country has broken away to form a new
state, South Sudan.  It is very complicated because national groups have in many places
been split up and have members in multiple countries.  An example of this is the Kurds
who live in significant numbers in Iraq, Iran, Turkey, and
Syria.

My own take on this is that national groups that
have been part of a larger state for a long time should not break away and become
independent.  This would, in my mind, be too disruptive of the international order
because it would potentially lead to ethnic groups splitting off into smaller and
smaller states around the world.  However, I would also argue that ethnic groups who are
being abused by the countries in which they live should be given the chance to break
away.

This is a very complex issue with no easy answers. 
Overall, I am in favor of ethnic groups learning to live together within larger states,
but I realize that this is not always feasible.  My answer to the question would be that
there can be no hard and fast limits.  Self-determination should be allowed mostly in
cases where the ethnic minority has been historically abused.