I think it takes 100 years for a name to “come round” again... once the original owners have passed on. Florence, Elsie and Edith were all popular in the 1910s, most of that generation are gone now and the names are coming back. By that reckoning, hard as it is to believe, in another 10 or 15 years, the likes of Doris, Joyce, Kathleen, Beryl, Irene, Muriel and Sheila, all popular in the 1920s and 30s will start come come into fashion (as their original owners die off), but we won’t see many in the vein of Janet, Susan, Pamela, Linda, Christine, Carol etc until approaching 2050.