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)
Source: http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7390052
Please also have a look at the ESA tutorials: