I would start after she turns 1 year old, but it is totally up to you....
I really liked the Tweedlewink DVDs, YBCR, LR and Signing time for my kids before they turn 1 year old.
I started to show Preschool Prep to my DD at about 15 months, she LOVED it and learned most of it in a matter of weeks! Their products are amazing!
Highly recommend.