Some real stats from my rigs.

12 Cards 306.9 Mh/s

Minute:   0.00001   
Hour:   0.00104   
Day:           0.02505   
Week:   0.17539   
Month:   0.75168   

So it's getting harder I used to get over 1 ETH per month. But if you view it as a hobby and enjoy it go for it.