How do I interpret this Japanese expression for "masturbation"? (Not dirty meaning)
So I noticed it s common for some Japanese people, angry, or foul mouthed or otherwise to use this expression.

Example:

1) I hate Vocaloids! They make the music we do look like nothing but masturbation!

2) **talking about a song they played** Just, it was too dark ^-^;; There, that is my masturbation time so I want to make it useful in many different ways in the future.

Japanese version of example 2:
ただ、暗すぎたな苦笑。あそこは俺のオナニータイムなので今後色んな形で活用していきたい

Can someone explain this expression to me? What would the narrow interpretation be? Is it like saying "I am being selfish"?

Question

How do I interpret this Japanese expression for "masturbation"? (Not dirty meaning)
So I noticed it s common for some Japanese people, angry, or foul mouthed or otherwise to use this expression.

Example:

1) I hate Vocaloids! They make the music we do look like nothing but masturbation!

2) **talking about a song they played** Just, it was too dark ^-^;; There, that is my masturbation time so I want to make it useful in many different ways in the future.

Japanese version of example 2: 
ただ、暗すぎたな苦笑。あそこは俺のオナニータイムなので今後色んな形で活用していきたい

Can someone explain this expression to me? What would the narrow interpretation be? Is it like saying "I am being selfish"?

Answer

When you have these questions on specific words, using a Japanese-Japanese dictionary is super helpful. My favorite Japanese dictionary is weblio.jp, if you aren't familiar.

http://www.weblio.jp/content/マスターベーション

From there you can see that you are right! It can be used to describe doing something that is just done to make yourself happy in a self-absorbed kind of way. We actually use "masturbation" in a similar way in English sometimes too, when we are describing selfish behavior, but want to make the point especially aggressively or emphatically.