What is the longest recommended time for a newborn to spend in a car seat? We can’t have an adult watch them in the back seat on a long trip as we also have a toddler. I’ve seen US research suggesting 2 hours in 24 hours is the max. recommended time. Thanks!

There is no specific time period, however when driving on a long trip, it is recommended by Road Safety organizations to stop & rest every 2 hours.
During this rest time, take baby (& toddler) out of the car seat & place them flat for sleep (if baby is sleeping)  or hold them in a neutral position.
Allow the toddler to “run around”. 

Red Nose has this information about the risks of baby in capsule/car seat for a long period.
Once the car journey is over it is very important that you remove baby from the car seat or capsule, even if this means waking baby, because it is not safe for baby to spend long periods in car seats, capsules or infant seats.
Research has shown that:
Babies left in a sitting position for a long period of time may be placed at increased risk for sudden infant death.
Car or baby seats may cause baby’s neck to flex forward which may block baby’s airway not allowing airflow
Falls from car seats used outside of the car as infant carriers are common, often involve children unbuckled in their car seats and represent a significant source of head injury for baby.
Never leave baby in carriers on shopping carts, counters, or other high locations
https://rednose.org.au/article/are-there-recommendations-for-car-seat-or-baby-seat-use