According to the Ergo website, their Infant Inserts are designed for a parent to safely carry the newborn baby from day one (if they are 7lbs or more) until age 4-6 months. It is designed to provide ultimate comfort and support for baby, by providing head, neck and spine support and ensuring baby is correctly seated while in the Ergobaby Carrier. They recommend that once your baby is able to hold her/his head up for long periods of time and is comfortable with her/his hips in a splayed position, the use of the Infant Insert is no longer necessary. For more information: http://store.ergobaby.com/ergobaby-faq#sthash.TkpRJ09L.dpuf
The weight are given as a guideline. You will have to see that baby is able to achieve knee to knee without having hyperextended legs then you can stop using an infant insert. Ergo baby infant insert is rather thick and hot, you can remove the back panel once baby have good neck support and just use the "seat" until baby is able to achieve knee to knee. Left is legs hyperextended. Right is the correct knee to knee.
If I remember correctly, I stopped using the insert when my baby was about 4 months. One thing is my baby dislike it... So I hardly use it also.
Reine Tsukino