I am sure that there have been books written on the topic of chemical
changes in oils on frying. It is very complicated chemistry. Your question
is difficult to answer because there are so many variables. The time at a
particular temperature is obviously important. So is the metal,
chlorophyll, carotenoid, polyphenol, and tocopherol contents of the oil,
and these would clearly vary from oil to oil. For this reason, I am sure
that some oils would be more stable than others and would decompose at
higher temperatures. However, it is likely that changes in most oils would
occur even at relatively low temperatures so it depends on what you mean by
"decomposition". For instance, some of the volatile constituents of the oil
would be lost at perhaps even 50-60 degrees and, depending on the
analytical conditions used, and the definition of decomposition, changes in
the fatty acids may be detected well below 180 degrees.

Alfred Poulos