It is generally recommended to take breaks and stretch every 20-30 minutes when sitting at a computer for long periods of time. This is because sitting in the same position for extended periods can lead to muscle strain, poor circulation, and other problems.
However, the exact amount of time you can sit at a computer without taking a break will depend on a variety of factors, including your age, physical condition, and the type of work you are doing. Some people may be able to sit at a computer for long periods without taking breaks, while others may need to take breaks more frequently.
To reduce the risk of problems associated with prolonged computer use, it is important to sit up straight, use a chair and desk that are properly adjusted to your height, and take breaks to stretch and move around regularly.