With each agency it is different. Some require 2 years other it can be 3 or 5.
Here is what it says on the Missouri DSS:
Who Is Eligible?
Children in the custody of CD who are legally free for adoption. Any family (parent at least 21 years of age) or single person (21 and over) in Missouri may be considered as a prospective adoptive family.
http://www.dss.mo.gov/cd/adopt.htm
Notice it says nothing about years of marriage. You could call and ask.
I would basically start with calling around the agencies and asking what their requirements are (age and length of marriage).
Now I can say this from experience the longer that you are married to more appealing you are to a prosective birth mom. It shows that you are stable and committed. Just my 2 cents.
Good luck! If you ever want to vent or chat you can pm or talk to us here in the adoption corner. The adoption world is very scary place and you need friends in it!