How long will mild gastritis take to heal?

Not long: Usually less than a week.

It depends: depends on the reason for your gastritis. Helicobacter pylori infection can be treated with a 10-14 day course of antibiotics. Medications such as ibuprofen, aspirin, naproxen can cause gastritis and while you are taking these you would continue to have gastritis. Acid suppressive medications will help, and usually healing is noted within 1-2 weeks