Brad, the 1910 census indicates that the Millers' (oldest?) son, Harold, was born c1895 in Wyoming. A possible match in WWI draft cards (1917) is for Harold John MILLER of Laramie, whose birth date was Sep. 1, 1895; his birthplace is listed as Cheyenne. Assuming this is a match, you could extrapolate that Fred and Mary were married sometime prior to January 1895.
I see a March 1, 1919, news item in the Wyoming State Tribune-Cheyenne State Leader, titled "Mr. and Mrs. Fred Miller Celebrate Wedding Anniversary." Neither the number of years married nor the wedding date is specified in the text.