def myfunc(rolling_window) :
count=0
n=len(rolling_window)
if rolling_window[-1]!=0 :
    return 0
else :
    for i in range(0,n) :
            if rolling_window[n-i-1] ==0 :
                count=count + 1
            else :
                break
    return count
df2['zerocum']=df2.groupby(['customerParentID','forecastGroup'])['demandQuantity'].shift(6).rolling(window=36,min_periods=1).apply(myfunc)

df2['zerocum']=df2.['demandQuantity'].shift(6).rolling(window=36,min_periods=1).apply(myfunc)