Originally Posted by
wphamilton
"It depends" isn't wrong, but for a more useful answer a Shimano cartridge bottom bracket can be expected to last 10,000 miles, and I'd personally expect quite a bit more than that. That would be near 10 years at the weekly mileage you mentioned.
I agree, the Shimano cartridges are a good value and should easily last several seasons. But we don't know that the OP has a Shimano cartridge. It could be one of the less reliable units (I'm looking at you, YST), in which case a much shorter life is likely.