How long should he remain hero?

Let him be a hero for as long as it takes. Its his time to shine.

I don't think he should be a hero, but neutral like Deathstroke.

He has to turn back to a villain at some point. He may stand on neutral ground but who knows how long that will last?

He's not a hero, he's just looking out for Lex.

He is scared of Anti-Monitor so much that he chose to side with the JL. Once AM is gone, Lex is evil once more.

I think putting him in one side over or the other is limiting him.
Have him be himself, manipulative,egocentric, power/fame hungry Lex who will do anything to prove he is the top dog and feed his inferiority complex.

Lex Luthor can NEVER be a hero. At most, he'll help heroes for self preservation. He's not a misunderstood hero; he's not Poison Ivy, Captain Cold, Sinestro, Harley Quinn, Bane or Jason Todd. He is a self centered, sociopathic narcissist who places little to NO value on other human life, or life in general; period.

I won't be surprised if Lex goes back to his old ways within the next year or two.

I'm sure Lex is up to something, he's already figured out one fairly big secret about a JL member. He's only there for his personal gain.

he's always at his best when he is evil.

I say keep him "good" as long as the joker's been away. Then, when no one expects it POW!

But in all honesty, this should be the turning point where he thinks he's doing the world a favor but really he's just a deluded bad guy.

He is still villain. The only reason why they are keeping him is to keep an eye on him. Right now he is just there trying to boost his ego and is staying due the Justice League wanting to keep an eye on their foe who already holds the secret of who Batman is. I'm sure Lex had something up his sleeve.

Lex never leaves to be a villain.

I always thought he was something in between.

I like him better as a villains :)

It's only a matter of time until he goes back to a villain. He has been, and always will be, an evil villain that is always looking for what benefits him. He's definitely playing the Justice League right now.

I prefer him being the type of character that will do what needs to be done to further his own goals but not necessarily a straight up villain. Many people walk over others in life to get ahead and get recognition and they do not ever understand the significance of their actions. Luther does, he just does not care if the ends justify the means. Literally he is like many of the heroes currently in the MU and DC universe he just has the villain label because he was the nemesis of a labeled hero (IMO).