After Austin was born we wanted another one right away!
I knew though that since I had no PP AF I probably wouldn't get pg right away, and that it was best to wait at least a year so that being pregnant wouldn't conflict with his nursing (since some babies wean during pg).
We thought that 18-24 months apart in age was the perfect age for our first 2 kiddos. I'm due 8 days after Austin turns 2, so we got our 24 month age bracket pretty well. Most moms I know say that 2-3 years apart is their ideal age, and this is what we want.
Long run: After Hunter is born we plan to TTC in order to have Hunter be around 3 years old when #3 will be born (mostly because we can NOT have 3 kids in this house we're in now, and we plan to build our dream house after this house so we need time to get money & building plans together). Then we want #4 to be about 2 years younger than #3. (Hope that all makes sense). This way Austin will be 7 when our youngest is born so we'll have 4 under age 7 which sounds perfect to me.
I know this is an ideal plan for us, and that God may have different plans for my fertility.
I think that 2-3 years apart per kid sounds pretty good. I don't think I could do it with kids less than 18 months apart, and for sure not less than 15 months apart!
I've heard too that adoption processes can take up to 2 years, so you may want to look into that soon to get all the info you can before starting that process.