The answer to this question (Does IUPAC nomenclature have the ability to name all organic compounds?) which references the complex biomolecule Maitotoxin which has a ridiculously long systematic name.
I doubt that this is the longest systematic name ever used for a synthesised and fully characterised compound. What is?
PS I'm sure we can name many proteins using stupidly long names, but I suspect that is cheating. Let's stick to lab-made compounds and the published literature.