Final answer:
Longitudinal research is the research design that takes the longest amount of time to complete as it involves following the same participants over many years or decades.
Step-by-step explanation:
The type of research design that typically takes the longest amount of time to complete is longitudinal research. This design involves tracking the same group of individuals over an extended period to observe changes and developments. Longitudinal research tends to be very time-consuming because it may span several years or even decades. It also requires significant financial resources due to the extended nature of the study. Unlike cross-sectional research that compares different age groups at one point in time, longitudinal studies track the same individuals' changes over time, making it easier to observe developments and lifespan progressions. However, the immediate applicability of the findings from a longitudinal study is usually delayed due to the extensive duration of these studies.