This is a very oft discussed issue. It probably could last 15k and not result in catastrophic engine failure, but 7500mi (basically 2x as often) is a reasonable safe margin.

Oh Oh Oh Oh I'm so happy because today I drove 30 miles to the nearest Autozone and found the German-made Castrol 0W-30 I haven't been able to find!!! Yanked all 6 quarts off the shelf!