I used to use flash cards when my baby was seven months old, but it did not work well on my baby.
When my child was in Pre-nursery, I asked the school teacher during a parents' meeting whether she agreed with flash cards. The school teacher said she was even sent by the school to Singapore and attended Glenn Doman's talk, her views on flash card was that it was lack of two-ways communication between the child and the parents. I was not recommened to use. The teacher said I can use the word card to help the child to learn/recognise words instead.