thank you, @heinzollerketchup.
Generally, the longer the temporal baseline (=the time between two images) the higher is the decorrelation which leads to small coherence values. It also depends on the cover of the analyzed surface. Bare ground mostly shows slower decorrelation than one with high vegetation cover.
As you can see here (tested with S1 data), coherence decreases from 0.7 to nearly 0.1 within 6 12-day-cycles (12, 24, 36, 48, 60 and 72 days, blue to red)
Luis Veci made nice tutorials which explain the whole interferometry approach a bit more:
I can also highly recommend the SAR EDU page which provides excellent material to numberous SAR topics for free: