I think it depend on how you study and how many time you spend on the process.
I didn´t learned by hard, and I started when I was very little (11 years old). It took me maybe one year to learn the basics, but I remember I could associate something but couldn´t figure out others.
The other advantage to learn with an astrologer is that you may ask whatever yo don´t understand, while if you do it alone, it may take longer.
The best way to master astrology for me is to try interpreting charts while you are in the process of studying , maybe after you know all tha basics.
And, yes, you are always learning new things...
To sum up...
It´s best to study with an astrologer, or somebody who is more advanced.
Investigate on your own.
Read interpretations done by others.
Interpret your own chart and the people you know.
Once you masted the basic, add more objects and details, variations to your charts (add chiron, nodes, asteroids, etc)
Check everything, try before you make a conclusion, don´t believe blindly in everything you are told.
Be optimistic, and believe in free-will.