I am planning an hour and a half ride with my 1 month old baby girl. I heard that being in the car seat for a long time is not good for newborns.
Is an hour and a half too long? Should we do a stop in the middle of the trip?
These seats are designed to be newborn-friendly. Any of them causing any physical damage to a child is unthinkable. Such seat should never have been made and accepted for sale. Unless you bought the cheapest seat on a flea market, you're going to be ok.
Long trips are discouraged because your child is restricted to the same position for their duration. If a baby is placed in an unnatural, non-physiological position, it is going to stay in that position for a certain time. You should make sure the baby is placed in the seat comfortably, that it's back and head are supported, that there are no unnaturally bent legs or hands. If something is wrong, your baby is most likely going to let you know - through crying, of course.
If you're really worried, make 5 minute stops every half an hour.
1,5h is not that long time. I've friends who made 4h trips on a weekly basis which started when their son was about (less I think) one month old. Their LO is 2 years now and has no seat-related health problems.
I've done a similar car rides with my daughters at that age several times. I don't remember there ever being any incident, nor damage to the girls. They are now 5 and 7, healthy as can be.
The infants are in their cribs napping for far longer, in a more "dangerous" position (less spinal support, danger of choking on vomit - not danger in the trauma sense). If the infant burps up in the car seat, clean it to ensure that the airway is clear.
The major reason not to travel with a newborn (< 3 month old) is not so much the carseat, but the immune system. A baby under three months old does not have a very well developed immune system, and so traveling to another location (particularly to be around new people) increases the risks that he/she catch an infection which is potentially much more dangerous to a very young baby than it would be over three months of age. (Yes, location matters - you can catch things from people, OR from the environment, and even if it is a person to person disease, you're more likely to be able to control things in your own home.)
However, if you need to travel for whatever reason, one and a half hours should be fine. This list is a good list to start from with regards to road travel; you will want to pay attention to #5 and #6 for this short of a trip. Sun protection is a must, but at < 3 months old you cannot use any sort of sunscreen, so instead you need to have clothing covering as much as possible, and a sunshade on BOTH windows in the rear seat plus the rear window (the far window can still allow in a lot of sun at certain times of day, unless you live in the mountains). A mirror is also highly recommended so you can check on baby without pulling over (as long as you're able to keep your checks to safe times and durations).
In terms of stops, this article recommends stopping every hour unless the baby is asleep, and trying to keep to his/her regular feeding schedule. I would guess 1.5 hours is still fine for going without a stop, but I wouldn't go much over that.