Some guinea pigs get very restless when out for more than 30 minutes lap time. My old cavy Carrot (RIP) used to sit on my lap for about 20 minutes before he'd want to wander off.
Another problem is that guinea pigs need water. If you have them out for more than 45 minutes then it's unfair if they haven't had a drink recently before you got them out.
When it comes to floor time either give them 2 sets of 30 minutes every day, or let them have water bottle access and keep them out for the whole hour.
Generally, a shorter lap time twice a day will be preferred for your piggies, but it's not a big deal for most cavies if not.
Remember, they still need time to settle in if they are that new.
Are you keeping your piggies outside? (I noticed you said hutch). If so, please bring them inside. -Wild animals: A hungry, determined wild animal will not give up until they get into the hutch. 15, maybe 20 minutes of chewing, scratching, pushing may be all it takes.
-Less attention: Out of sight, out of mind. Even if not out of your mind, will you be going out there in pouring rain/freezing/boiling weather every few minutes to check they are okay, and talk to them?
-Weather: The weather alone is enough to kill a guinea pig. Cavies cannot deal well with hot weather. They can deal with cold more than hot, but when the temperature drops under 15 degrees Celsius/60 degrees Fahrenheit your guinea pigs will be very cold, and under risk of illness, or worse, freezing to death. If it's super windy, will your hutch be sturdy enough to take the blows? And even if it is, won't the wind be bashing on the hutch, creating a terrifying environment for the pigs? Rain will soak the hutch, and possibly the pigs. The hutch covers they offer barely have any ventilation, which is something guinea pigs must have.
There are many more reasons, but guinea pigs are generally so much healthier and happier indoors. Find somewhere to fit in a 7.5sq ft cage at least so they'll have enough room to get away from each other when they need to and popcorn and run laps. You can build a C&C cage to give them this spaces which are cheap, versatile and easy to make. More information can be found on this website: http://www.guineapigcages.com/