I notice that the rewards gone down since i joined 3 weeks ago, It would be much painstaking to get penalized in mining and repaying it will be longer this time.
No, I believe that penalty roughly equals to 10 hours worth of mining and that relation doesn't change with epochs passing by.
When there are too many nodes online the mining rewards will go down. Yesterday my 12 hours mining only manage to collect 32 Idena. The week before i can get atleast 60 - 65 Idena
With 343 valid identities / mining rewards = Idena mining rewards per nodes