The length of time it takes to adopt depends upon the circumstances of the adoption. 

Generally, it takes less time to adopt older children and biracial or special needs infants than it does to adopt healthy Caucasian infants.  With regard to healthy Caucasian infants, the waiting time is shorter with private agencies than with public agencies. 

The quickest way to adopt a healthy Caucasian infant is to combine the resources of a private agency and the couple's own independent search efforts in the form of advertising and networking. 

In my experience, couples who do so can probably expect to adopt within a year.