Please Read - FORUM RULES!
As you improve with practice, you can quickly get it down to 30 minutes per song or less. A highly skilled audio mastering engineer can master a song in 10 minutes. At least that's how long it takes me. Note - I ALWAYS split my audio mastering into two separate sessions so I can finalize on fresh ears. I talk about this later in the book.
Some of you might think 10 minutes to master a song is too fast. It is if you're just starting out. You will need more time to A/B compare and to test out different settings through trial and error. But in time, you'll learn how to quickly diagnose what a mix needs and what to do to achieve your desired results.
Now (after many years), if I spent 10 minutes or 40 minutes to master a song, my results would be exactly the same. An extra 30 minutes would not make my masters any better.
A good non-music example would be tying your shoes. The first week you learned as a child, it might take you a minute to tie each shoe. And you still might not get it right! Now, you can tie both shoes to perfection in 5 seconds!
If you spent an extra two minutes tying your shoes, could you do a better job? No. Whatever you can do in five seconds, the results will be exactly the same as if you spent two minutes.
The same goes for a highly skilled pro in audio mastering.