When do te think Arthur is gonna find out about Merlins magic?

I think Arthur might find out in S.3/4 & they'll have an argument but after Arthur calms down he'll realise Merlin only uses his magic for good deeds and will accept Merlin as he is and they will still be friends.

I think Arthur will find out about Merlin's magic either late S3 o S4... I think Morgana will be wreaking havoc in Camelot o something, and Arthur and Merlin will agree that the only way to combat her is to use magic against her... and Merlin will have to come clean. Then, of course, they'll be conflicted about fighting her... but that's a diff story. So then once Arthur knows, he'll be completely shocked, a little mad that he has known Merlin for so long while yet not knowing him at all... but they will reconcile and then combine their strengths to be awesome!

Well I believe it would be later on Season 3 o 4.
Knowing how Morgana is going to revenge Uther for whatever hes done to her,and Camelot would be close to its end,when she and Morgause would come for their final strike.
I believe Merlin would speak the truth to Arthur,before leave his chambers to go and fight with the rest of the Knights outside,doing their best to protect the fallen Kingdom.

M:There's something I have to tell you..I wouldn't do it any other day,but I fear todays it's the last one,so I..

A:Merlin stop mummbling..Besides who detto it's going to be the last one?Gosh you're so dramatic..Yet there's one thing I would like to give you. *throws him a sword which Merlin caught it in surprise.*

M:Im not going to need it.*drops the sword.

A:Allright,listen..I know te look riduculous with a sword and probably you're going to kill our knights and not Morgana,but te worth a try.

M:Arthur I dont need a sword cause Im a sorcerer..

A:You're what? *burstes in laughing.*

M:You dont believe me?Then I will mostra you.

And magically all the things in the room starting flying.
Arthur stops laughing and looks Merlin with a outraged look.

A:We're going to talk about this later.Now do your best,and hope we can win Morgana,with the way my father never wanted.With magic....