I believe it is variable. When you attack planets, the damage you inflict can sometimes just kill civilians for a while before any facilities are destroyed. Sometimes facilities start getting destroyed soon after you start orbital bombardment. So I can't say that there is a known specific amount of damage that facilities can take.
This is how I see it, if I'm wrong, someone please correct me.
Of course, I don't believe in orbital bombardment. That's why I'm a member of Intergalatic NABOB (NAtions Banning Orbital Bombardment).
I rather seize and plunder a rival's planet, much more satisfying.