The standard manufacturer's suggestion for replacing the blades on an electric razor is every 12 months. This recommendation is found on the instructions for Norelco, Braun and Panasonic. The manufacturer wants the consumer of its products to be satisfied with them and therefore believes the blades must be replaced annually to maintain the level of quality of the electric razor.
While the manufacturer has the greatest knowledge of its own product, it also wants to make profit as a company. The replacement blades are made by the same companies that sell the electric razor. Recommending a quicker replacement than necessary would increase sales for the company and they would profit at the consumer's expense.
The frequency to replace blades in an electric razor will vary by person. The annual replacement guide is more of an average that does not address each user's situation. When the razor seems to be losing some of its effectiveness and the user can feel a pull on the hairs instead of a clean cut, then the blades should be replaced.