I also try to sharpen blades at least once a week. I would only sharpen them more often if I accidentally hit something. I always try my best to be careful when mowing, and not to hit anything that could damage them or make them dull quicker.
Part time, every 10-12 hours. Even then the blades usually aren't THAT bad, but we sharpen them anyway to make sure they are perfect.
Also scrape and service mowers every 10-12 hours. We end up doing it once a week.