I have a household of 4 adults and 3 kids currently (a month ago was 4 kids and a few months prior was 5 kids).
I can honestly say that you simply HAVE to schedule it in. Everyone has different needs for amounts of alone time-and you have to work through those details as a group. But no matter what amount of time you are talking about you have to schedule it in.
My suggestion would be to sit down with the other adults and let them know what you said here, and that you need to find a good time each day for a 1/2 hour of quiet time and once a week or so a few hours to yourself. Work together to figure out when the best time for that would be.
I take a bath for 1/2 hour or so every day as soon as hubby comes home while he keeps our kids busy. Every morning I take a walk for 1 1/2 hours with my boyfriend and the youngest child (in a stroller) who usually sleeps. I generally wear my headphones, so I CAN talk if I am up to it-or I can walk with them and only listen to my music if needed as well.
My hubby tends to use his 45 minute drive home for down time and the boyfriend takes about an hour or so every night after hubby and I go to bed.
Just depends on what works for each person with the kids (and of course it gets screwed up if kids are sick or something like that!
"Love As Thou Wilt"