Retraining a machine learning model
It depends on a lot of factors:


Does the function you're learning change overtime ? If you try to detect dogs in pictures, and you already have a good accuracy, then relearning is not necessary. But if you try to detect topics in news streams, then you'll need to update your model regularly. 
Do you have initially enough data ? If you receive a stream of training data, then you should find an algorithm that can be trained in an online manner. 
If you are looking for constantly improving the accuracy, then the more data you feed the better results you'll get. Regularly retraining a new model with the old data + the new ones might be sufficient.