I want to know if there are any guidelines for how long a baby can be in a car seat during a car trip. I am aware it is not recommended they are left in a capsule for a few hours but is this the case for a car seat also?

The general recommendation from safety organizations is 2 hours, particularly if sleeping, is long enough for a baby/toddler in a car seat/capsule.

Red Nose has the following information.
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
https://rednose.org.au/article/are-there-recommendations-for-car-seat-or-baby-seat-use

So if you are driving and your infant falls asleep in the capsule or seat, is it safe to finish the drive or does the infant need to be woken straight away?

The concern is for long periods in car capsule/seat,

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.

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
https://rednose.org.au/article/are-there-recommendations-for-car-seat-or-baby-seat-use

If you have a long journey such as an interstate car trip, is it sufficient to take the baby out every couple of hours for a break then keep driving (eg. A three - six hour trip).? Or is there a maximum cumulative amount they can be in a carseat per day?
Also at what age is this no longer a concern?

There is no exact answer to this, but I am unaware of any research that supports or denies any cumulative effect.
The recommendation si similar to the rest breaks for adult drivers - take regular breaks.

Also, there is no definite age cut off, but as baby grows & can move & lift head & change head position, the risk reduces…..especially when baby is in a forward-facing car seat & at the appropriate age & development.

A long drive, without a break is not generally recommended for anyone.

Baby Car Seat Guidelines

How long can a baby spend in a car state. If you have a long journey such as an interstate car trip, is it sufficient to take the baby out every couple of hours for a break then keep driving (eg. A three - six hour trip).? Or is there a maximum cumulative amount they can be in a carseat per day?
Also at what age is this no longer a concern?

ChatGPT
When it comes to traveling with a baby in a car seat, it’s important to prioritize their safety and well-being. While there isn’t a specific maximum cumulative amount of time that a baby can spend in a car seat per day, there are some guidelines and recommendations to consider.

For infants and young babies, it’s generally advised to limit the time they spend in a car seat to minimize the risk of positional asphyxiation and to allow for proper development. The American Academy of Pediatrics (AAP) recommends that infants spend no more than 2 hours in a car seat at a time. This recommendation includes both car trips and other situations where the baby may be sitting in a car seat, such as strollers and swings.

During long journeys, such as an interstate car trip, it’s still essential to take breaks and allow the baby to have some time outside the car seat. Taking breaks every couple of hours for the baby to stretch, play, and have some tummy time is a good idea. These breaks can help prevent discomfort, promote circulation, and allow the baby to engage in age-appropriate activities.

As for the age at which this becomes less of a concern, it varies from child to child. As babies grow older and gain better head and neck control, their risk of positional asphyxiation decreases. Generally, around the age of 6 months, when babies can sit up with support and have better muscle control, the concern about prolonged car seat use diminishes. However, it’s always best to consult with your pediatrician for specific guidance based on your baby’s development and individual needs.

Remember, the primary goal is to balance the need for safe travel with the baby’s comfort, development, and well-being.

Red Nose information states the following:
Red Nose has the following recommendations about baby sleeping in a car safety device.
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.
https://rednose.org.au/article/are-there-recommendations-for-car-seat-or-baby-seat-use

There is no specific time that represents “long periods”, however when driving for extended periods, it is recommended by Road Safety organizations to stop & rest (& stretch) every 2 hours.
During this rest time, take baby (&/or toddler) out of the car seat & place them flat for sleep (if baby is sleeping)  or hold/lay baby in a neutral position. Allow a toddler to “run around”.
After 2 hours many newborn babies will also require feeding.
I am unaware of any cumulative effect.

This information is specifically related to baby less than 12 mths - the age at which Sudden Unexpected Death in Infancy(SUDI) no longer occurs, but may be relevant for some older toddlers/children.