my friend was told to wait at least seven days - i think whilst it is 'effective' straight away - you're best to try and let your body be on the 'safer' side.
Especially as with breastfeeding - although they say exclusive for six months is protection - you can't know that you're definitely not ovulating - and then just in case you have ovulated seven days is enough of a safety net - if you get what i mean??
Obviously not a medical opinion, but HTH