A certificate program can take from 4-8 months to complete, depending on the amount of time you devote to the course. Most certificate programs are self-paced which means that you train according to your schedule. If you only have time late at night, you can study then. There is no set time you HAVE to be at the computer or in a classroom. You can study as quickly as you want to.
A degree program will typically take 2-4 years to complete and will cover broader subjects. The average time it takes to complete an associate degree is 2 years and a bachelors degree takes about 4 years.
Overall your personal preference and/or schedule demands should be the deciding factors. Remember that you can find quality education online. AHIMA has approved many online medical coding and billing schools.