How long did your baby last in the infant carrier

I was at target today and the chicco nextfit convertible car seat was only $125 so I went ahead and bought it. $200 off the original price! Who could pass that offer up? I was thinking that I want to get a travel system as well but now I'm not so sure if I need the infant carrier. I know an infant carrier would be way more convenient but how long do they really stay in them? How many months was your baby when you had to switch over to a convertible car seat? Is it worth it to buy the infant carrier as well or should I just get a stroller now?