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Assuming a proper maintenance schedule, we can reasonably reduce that time window.

The article indicated the crack was found not during a regular inspection, but during modifications: "Boeing notified the agency of the matter after it discovered the cracks while conducting modifications on a heavily used aircraft."

And they only found the cracks in other aircraft after specifically looking for it after they noticed the crack in the first aircraft, so it doesn't sound like something checked as part of routine maintenance.

How often are these particular frame components inspected for cracks or wear?

I don’t know for these. C-checks for 737NG are performed every 730 days and include corrosion/wear checks for the most stressed parts, but not all parts (which are checked at D checks): https://en.wikipedia.org/wiki/Aircraft_maintenance_checks

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