In which epoch should i stop the training to avoid overfitting
As long as your validation accuracy increases, you should keep training. I would stop when the test accuracy starts decreasing (this is known as early stopping). The general advise is always to keep the model that performs the best in your validation set.
Although it is right that your model overfits a little since epoch 280, it is not necessarily a bad thing provided that your validation accuracy is high. In general, most machine learning models will have higher training accuracy compared to validation accuracy, but this doesn't have to be bad.
In a general case, you expect your accuracy to behave in the following way.

In your case, you're before the early stopping epoch, so even if your training set accuracy is higher than your test set accuracy, it is not necessarily an issue.