Right now I am a mother (largest job description in the world), and a have a side business of maintaining aquariums, took me about 4yrs to feel confident at my reefkeeping abilities and I am still constantly learning.
Before that I was a zookeeper. 4yrs of college to learn basically nothing about taking care of animals for a living, but necssary I guess. The zoo field is a constant learning environment, some areas take about a month to learn to do, but you need to constantly be researching and learning more about the animals, some area's take 6mo to be qualified at (usually hot areas so you don't end up killing someone).