Here is Glenn Doman's quote on the best time to teach your child flashcards,
"It is best to use the morning hours to teach. Afternoon is generally not as good a time, but in the evening things start to pick up again.
In any event, you choose those times when your child is bright and alert and avoid like a plague anytime he is not" Glenn Doman How To Give Your Baby Encyclopedic Knowledge
Learn how you can expand your child's learning capabilities, click here.
For more info about the "Glenn Doman Method", we would highly recommend you to read his books and visit their official website.