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?

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.
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.
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.