Follow the recommended dose and you'll be fine. 2-4 hours is typical for bedrooms and 4-8 hours for bigger rooms in a normal sized house. The problem comes when they are left running for days at a time.

Ozone works well for removing fresh smoke smell. You still need to wash soft furnishings like curtains, couches and carpets as well as all hard surfaces like the walls, ceiling, doors, windows and frames. If they've been smoking in there long term you'll never get the smell out. Nicotine, tar and all the other nasties get absorbed into every surface.