Hmm...this is actually a bit of a two-pronged query... I find that a lot of people are really asking about disposition when they mention temperament, so I'll address both... My own experience is that you can readily tell what your cockeral's disposition is going to be like, the qualities that determine whether he'll be a pleasant bird to have around and what he'll be like to handle and how he'll interact with humans, quite early on. You'll certainly know by the time their first adult feathering is in and their spurs show the first signs of starting to grow, so you should be able to weed out the true jerks long before they become dangerous.
Temperament per se may take a bit longer to assess. Most people with backyard flocks want a good flock guardian in their rooster, I think, and some of the sweetest roosters imaginable can be really irresponsible and kind of useless when it comes to looking after the girls. Most do show early signs of being good guardians and get better as they age. The odd one doesn't, BUT they can still be terrific pets if they're otherwise kind and trustworthy. It all depends on what you want of your rooster, I guess, and what flaws you're willing to forgive. The ideal, obviously, is the rooster who's both fiercely protective of and solicitous with his flock, yet charming and gentle to deal with when it comes to interacting with humans. For most backyard situations, I'd say the gentle with humans part is the most important, so concentrate on finding a bird with a great disposition first and foremost if it's important to you too and foster it if you've already got a promising bird by always treating him kindly and respectfully. Rough, painful handling can ruin a nice cockerel pretty quickly, by the way, so...fair warning there. Then again, I think people who mistreat animals deserve any and all retaliation they're dealt.
Recent example from my own chicken keeping: I've currently got two young flocks in that mostly feathered out into their first adult feathering but still not fully mature stage and I've been letting them out to forage around the back yard about half an hour before I let the established older layer flock out so that the youngsters can mill around the common area up by the runs and use the dust baths, etc, without being harassed by the gnarly old hens for a while. Last week I let them out one morning and the buff orpingtons immediately ran off to the other, far side of the yard to hang out in the rugosa hedge and forage through the meadow area there. The olive eggers, meanwhile, hung out by the runs and kept me company as I sat on the park bench in the common area. It's tomato season here and I'd gotten a nice big juicy one off one of my deck plants before sitting down and started taking a few bites out of it before I turned the remainder of it over to the olive eggers. Well! One of the buff cockerals, Don Martin, the greediest chicken in the world, saw that I had a tomato from a hundred feet away and suddenly came waddling back just as fast as he could. All by himself. None of the pullets with him. Could have cared less about their welfare at that moment and certainly wasn't interested in calling their attention to a possible treat...he wanted that tomato for himself! I wound up tossing him a piece, which made him very happy, but still...pretty irresponsible of him to just abandon his flock like that, I'd say, for no reason other than greed. Meanwhile, the second, subordinate buff cockerel stayed behind... Don Martin is a very kind, docile young rooster to handle, and great-looking and quite amusing and personable, and because of that he's going to be a keeper. But geeze louise! Immature and crappy about doing his job right now or what! I expect he'll improve with age, but right now the second buff cockerel, Nugget, actually has the better temperament in my estimation, plus he has just as nice a disposition. I expect I'll keep them both...
But I don't want to get too attached either if it seems like he might still go dark side on me someday. 
