Whenever I feel substantially undervalued. For my first few jobs, that was relatively often (1.5-2 years) since most companies will offer tiny raises, while your skill growth will be raising your value much more substantially. My first few raises were about 2-10% increases, while switching jobs was about 30-50% increases. Eventually the skill growth will slow on the critical path stuff (don't let it slow too much!!), and you might get to a level where promotions come with more substantial increases and then job hop less.