Breast pump valve/membrane replacement

Thy
Does anyone know how often MedelaSymphony breast pump valve membranes (thin white flaps) need to be replaced?  Couldn't find the answer on website or manuals.   I just replaced my Ameda Purely Yours breast pump valves that I use at home and got 1 more oz out of one side.  I read on the Ameda website and on forums that valves can affect suction and therefore milk  supply!  

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