There is no realistic way to capture anti-neutrons, so there is no direct experimental measurement of their lifetime. You can set lower limits in the microsecond range from collider experiments (a decay before they hit the detectors is very unlikely), but that is not very surprising. The lifetime ratios of various other particles have been measured, with very precise results (e. g. 10-4 level for muons) - all in agreement with CPT. It should be the same, and even if CPT is violated slightly, deviations should be incredibly small.