How long is the averaging length of the SNR estimator?

Looking at the documentation for “gr_probe_mpsk_snr_c” (
http://gnuradio.org/doc/doxygen/classgr__probe__mpsk__snr__c.html), it
says
it computes a running average of the signal mean/variance. How long is
this
running average? Meaning, if my SNR is changing over time, how much will
past values affect the current value?

Also, is there any discussion of what the alpha smoothing factor does?
In
the digital-bert example, it’s set to a magic “10/symbol rate” number.

Thanks.
-William