I would say that when it breaks off and flys into the jet engine intake, would be an unexceptable case of an aircraft damaged antenna. Other than that maybe it the VSWR is 10 or so?
I assume you mean longer term "wear and tear" effects rather than the "worst case" that GOTWW suggests above.
On one aircraft which carries our equipment, the UHF/VHF and IFF "shark fin" type of fuselage mounted antennas suffer fairly predictable leading edge wear over time. I am not sure if there are specific limits for this sort of degradation but it seems that, provided there are no structural fatigue cracks, the maintainers send them for repair, either locally by the avionics workshop or to the manufacturer. This seems to consist of filling the chips and dents and then repainting with non R.F - abosorbent paint (of course!)
