Well it essentially took 10 months!!!!!!!!
We would have moved the babies into Maxson's room sooner around 8 months but we were leaving on a 6 week trip at the end of the summer not long before they turned 9 months and didn't want to get them out of routine of their pack n plays.
Here's some big tips that helped us and a timeline of events that helped all the boys be ready for it.
1. For essentially 10 months Max was constantly old "Shh! The babies are sleeping!" "Don't wake up the babies!" This was engrained in his head. He'd had experience with crying babies in the car and came to know he didn't want to wake up the babies. This has helped so much when the babies are asleep in the room and he needs to go in or out.
(Bryson always throws his stuffed animal into Quinn's bed when he wakes up).
2. WHITE NOISE. Boom. Gotta have it with any number of kids, it just helps. We use this brand. It definitely gets loud enough and has a variety of sounds. We use only the waterfall or white noise sounds
3. Black out curtains. The rooms is not totally blacked out, but it's good enough. My SIL is a ninja at blacking out rooms, it's amazing.
4. We trained the babies from about 2-3 months to learn to fall asleep by themselves and sleep even if the other is crying. How did we get them there. You keep doing it no matter what. I totally would have been prone to separating them for naps to get them to nap longer since they sometimes woke each other but I literally didn't have a choice. They HAD to get used to sleeping through each other's crying, the white noise helped.
5. How did Max get used to sleeping through their crying? They always cry at some point during Maxson's nap, mostly when they wake up. When we had our 6 week trip Maxson had to be in the car with the babies happy or crying and he just learned to sleep through it. He used to be a light sleeper during naps but I think the car trip helped him get over it, and he plays super hard so he naps pretty hard.
6. We put the babies down to sleep around 7 and Max around 8/8:30. For both night time sleep and naps we give Max the choice to go to sleep on his bed or mommy and daddy's. 90% of the time he usually chooses his bed. Before our trip he probably would have always chosen our bed. Our trip pretty much left him without his own concrete bed all the time so coming back from the trip he was excited to get to sleep in HIS bed all the time and then add the babies to the room it just seemed cool for him.
7. I leave the door cracked during naps and totally open at night time. Then Max can come out if needs without making a ton of noise to call for our attention (we have a baby lock on the inside of his door still since really bed tantrums end up in his room during time out).
I guess mostly our kids have been forced to sleep through noise of each other. We don't have a house big enough to accommodate quiet sleeping arrangements all the time so they HAD to get used to it, even I didn't have a choice.
8. AND LAST OF ALL, sometimes it doesn't work. Sometimes they wake each other up and they only get short naps. Sometimes Max turns the lights up and wakes everyone up. Sometimes Max and the babies just play and play and play :)