We have five children ranging in age from 7.5 years to 13.5 months. Our youngest two are 6 weeks apart in age and came home from Guatemala 3 months apart.
We began the adoption process when our then youngest son was 17 months old and we wanted to make sure that he and the baby we planned to adopt were seperated by at least 9 months. Along the way we decided to adopt a second baby. The entire process from filling out the application to bringing home our youngest was about 20 months, but we also waited out the Hague in Guatemala during the summer of 2003.
Other than asking agency policy, I think you need to look at your own time and energy level. The closer you space your children, the more labor intensive and exhausting the first couple of years are. But it seems like after those first 2 to 2.5 years things start to get easier. At least I'm counting on that. But there are plenty of nights that I am ready for bed the second the kids are down.
We also try to make one on one time with each of our children. But that is getting trickier as they get older and are beginning to get involved in extra activities like soccer and scouting, etc..
Good luck in your decision! It's great you're looking into it now and have time to really research agencies.
Kim