Okay. I dont know much about bikes but a friend of mine sitting beside me who is a biker just gave me his answer. He changed his and he complained that it was because the tread depth decreased and reached the measure point. That was when he decided to change the tyres. Hope you understand. The tires gets practically bald and its a little too risky for a rider as it offers little to no grip on flat surfaces, forget cornering. So his advice is that you do not wait out till the tyres are completely bald. Also he changes the tires when he notices a change in handling. Many times on high performance motorcycles tires wear oddly and even though they may not be all the way down to the wear bars, handling can be effected. Just hope that helped you. And @velimir. I am sorry i answered on your behalf. I judt loved the post and wanted to contribute to @georgechuks question.