A decade between each is perfect! Well at least it has been for me. Funny thing is... I didn't plan it that way. But if I could do it over again, (and had any control of the outcome) that's exactly how I would space them.
I (we) have three girls 24, 14 & 4. My firm belief is that children are an heritage from the Lord, so in my case...they each arrived exactly at the right time. Plus we have built in babysitters.
All three girls are unique but have great love for one another in common. Do they have their disagreements? Yes! But it is more easily worked out because of the age gap (maturity and experiences) between them. The interaction is great to watch, I can trust them to "really work it out".
It's also a joy for me to be able to engage more fully (focused) on a one to one basis in their lives, without feeling pulled in too many directions. I love being able to sing lullaby's to one, give teen advice to the other and go out to lunch with the oldest who by the way is now married.
My husband and I have a very full life. We love spending time with our girls and our son in law. And having our girls10 years apart has really kept us young at heart. Will I have another in oh 5 more years to keep the decade pattern going? At age 47 now, 52 then... I think NOT! But then again it's not my plan.