More important than how long the carseat can last is how old, how heavy and how tall your child is currently. No point getting a carseat that will fit a kid when he is 5 years old, but the car accident happens when he is 5 months old but the carseat didn't fit him well at that age!

Are you asking for baby not born yet or baby already born? How old is baby, how heavy and how tall/long?

You have read from my earlier posts that once there is a car accident, you have to buy another carseat because carseats are all one-accident use only right?

Babies should NOT be left in the carseat for longer than 90 minutes each time, BECAUSE the carseat is not lying flat. Babies should lie flat most of the time. However, lying flat in the car is NOT safe if there is a car accident, because the 45 degree angle as per your carseat's indicator, protects your baby's entire spine in a car accident, like a rocking "cradle when the bough breaks" in the song rockabye baby song.

So just tell your auntie that baby will most often be in his crib/floor/cot lying flat at home, not in the car. BUT this angle is necessary in the car. If you have to drive for more than 90 minutes with your baby, break the journey up so that baby gets out of the carseat for half an hour or so every 90 minutes.

Once your baby has head control, you can extend his/her time in the carseat to two to four hours, but every two-four hours, still need to let out to stretch and move about. In fact, adults should also get up/out and stretch/walk around every two/three hours to extend lifespan.

More info: http://www.babycentre.co.uk/x554834/is-it-...in-his-car-seat

You can also tell your auntie that baby must NOT be fed lying flat. Babies also need the 45 degree angle for feeding to make sure that if they vomit (reflux is very normal for babies), they don't accidentally swallow back their vomit and choke.