well it first depends on the type of discus... blue diamonds and some of the ocean greens like 2 years.. but I had some canary yellows that bred in 11 months... some of your pigeon bloods will spawn fast too, and also your blue turqs.
now wild discus they might be ready in 1 year, or 2 never had the chance to breed any, but most are already to size.. but i think they need lower ph water than the standard tank raise discus like 5.0
remember do water changes like 2 times a week, and feed not a large amount of food, but a bunch of feeding per day... small fish 2-3 inches feed 6-8 times a day.. 3-5 4 times a day.. and 5-7 2 times a day. breeding pairs I only fed once a day.. and then after they had fry, I would feed them about 4 times a day to make sure they would keep producing slime for the fry....
andy