If I like them enough I usually rate them whenever I finish the latest chapter I can read (but if it's so bad I can't continue, rate it automatically as 1 or 2 lol). For those which does not give me the best impression while reading but got the chance to read it till the end, I'll give them at least 3. If ever I changed my mind from 5 in the middle but became somewhat 2 in the end, then atleast 3.
Usually i wait 'til it´s finished but if it´s incredibly good and the story it's about to end i might rate it at the spot.
But i have noticed that if it´s an ongoing story my perception tends to be more positive than when they are already finished (this i noticed after rereading several mangas that i followed while they where being uploaded and that i really really liked, but after i read them again when they got completed they just where not as good as i initially percibed and ended up as a dissapoinment)
sorry for the super long answer!! (⊙…⊙ )
Just wondering but do you rate a manga before its completed because its good or do you wait till its completed, like for yes dislike for no or just write (づ ̄ ³ ̄)づ