I broke an arm bone, l done an open reduction surgery with screws&plates for like 6 weeks now, how much time does it really need to heal?

About 8 weeks. Usually if a radius or ulna fracture is treated with plates and screws, it takes about 8 weeks to heal the surgery is performed to obtain the best longterm result and obtain immediate fracture fixation.
3 months. Although every fracture is different, a standard type fracture of the forearm is healed enough at 6 weeks to start strengthening exercises. These strengthen both the damaged muscles, and the bone. There continues to be some improvement for nearly a year.
Longer. Hard to say without more specifics. But, most bone healing should occur over the first 3 months. Other things such as strength, endurance and coordination can take much longer.