Hi there
Yes I'd aim for the longer A time. Guidance time is 1hr 45 - 2hrs at 4 months so perhaps if she is used to having a short A time of only 1hr you'd need to increase over a number of days rather than a big jump. You might say for 2 days that you are going to aim for at least 1hr 15 for instance, if she isn't showing tired signs then you can keep her up longer but if she is then only stretch to 1hr 15. then after a few days increase to 1hr 30 and then to 1hr 45.
It's okay for the A times to vary across the day a bit. If she is happy to do 2hrs that isn't a problem it's just I wouldn't stretch all the A times to 2hrs in one go as she's having some of only 1hr at the moment.