That's not how they work.
When a bio-wheel is fully cultured, it won't have 1/100 oz. of bacteria on it. It will have several types of algae, dirt and general gunk(technical term). The amount of bacteria and the time it takes to cycle depend directly on your bio-load and filter sysem.
Bio-wheels don't need to be replaced unless there is physical damage. Bearing surface wears out, paper in wheel deteriorates or damage incured during maintenence.
The bio-wheel can & will slow down or turn erratically (out of balance), this is caused by improper maintenence...Algae and technical gunk have built up faster on one side of the wheel, they just need to be rinsed in used tank water and if it doesn't come off use a soft bristled art brush to remove the build-up being careful not to damage the wheel.
Hope this gets you back on track.
Dr Joe
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