Dewa,
My one seems like not much brake dust on my rims wor......there is no standard of when to change the brade pad geh cos this is a wear and tear item ......finish than change ...normally i will change it while it reach about ~70% usage than change .......just check at the shop and compare with the new pad to judge the usage or ur attached brake pads.
Cheers....Lancer Forever...!!!...I am back !!