Rendering of a scene: 4-5 hours for this?

Day

 Night

After a 4-5 hours rendering session, I optain those guys (4 hours for day and 5 hours for...)

this...


So yeah... There is something I'm doing wrong. I really should learn how to master the render passes, otherwise, I will never be able to do anything in the future! Imagine my final projects! It could be a mess...
Fortunatly, I learn. I am removing at this moment the white flowers (which look like ugly ice flakes.)