It is known that we don't know if our fight in an infested system was successful. Beside this, are there any other clues that show the retreat of Thargs in an infested system empirically? Fading of higher NHSS? Increasing aggressiveness of the remaining scouts? Fading of number of NHSS? Anything?
Basically, you cannot measure the progress, but achieving the target: Once an infestation is cleansed, all NHSS stop to appear. As long, as there are NHSS, the infestation has not been cleansed.
Also, AXI waits for multiple pilots to reports no NHSS in a system, as they sometimes do not appear for single CMDRs to due other reasons.
Edit: One issue, if the system is too close to the Pleiades, it will generate NHSS independent of the infestation state and there is no way to confirm it as cleansed.