We've been using ours for walks & stuff for 1-2 months maybe? She is still in her infant seat in the car though, so when we're out & about that goes into the snap n go stroller still.
We've been using our Chicco Liteway stroller since she was about 4 1/2 months. She's still in the infant seat but we started using our Britax B-Agile without it (unless she's sleeping) around 5 months since she likes to look around.
Re: Big girl stroller