I assume that by investment, you are referring to the overall effort of developing the skill sets to excel within your chosen profession. This is a ongoing/continuous effort that should be part of your daily life throughout the duration of your career. There will be more of an emphasis on learning/training early in career or when you enter something new, but you should always look for opportunities to develop. Let me know if there is a more specific question you would like answered
Investment should be a part of your career from the 1st day of your job. If someone is not considering about investments, he/she is most likely to face difficulties fighting inflation in future. Investment is not only about fighting inflation, it's about creating incredible wealth in long term using the power of compounding.