I guess that would depend on what you do in the TM and how much you use it.
I use mine lots but I don't usually do much grinding like grains or ice.
I mainly use it for kneading, mixing, steaming and cooking.
And now I have 2 bowls that means that each bowl is doing half the work of what only having 1 bowl. So maybe I will get 6 years out of mine before they need replacing?