How long can i use the same water bottle before washing or acumulating harmful bacteria?

Less than a week: First, make sure the bottle is bpa free and avoid reusing commercial one-time use water bottles over and over. Keep the water bottle out of hot environments and rinse it frequently. I wash mine with soap and water every 2-3 days.

Wash it twice a week: A healthy person using a water bottle to carry clean drinking water is unlikely to get an illness from "harmful" bacteria in the bottle. Neither the water nor the bottle are sterile, so there will be some bacteria, but probably won't cause disease. Rinsing the bottle with a little drinking water at each refill will help keep the bottle clean. Actual cleaning of the bottle can be done twice a week.