How long it will take to become a CSI depends on where you start. The quickest route would be to learn a specific skill related to forensic evidence such as fingerprint analysis, ballistics, or photography and then get a job working with an agency as a crime scene technician. After proving yourself and becoming a sworn officer you may find yourself investigating a crime scene fairly quickly.
If you prefer to go to college (which will increase your salary and your chances) then it will take longer. Many CSI’s have a four year Bachelor’s degree. Some pursue a Master’s degree which will take at least two more years.
Some CSI’s began as police officers and worked their way through the ranks over many years. If you are a police officer, take every opportunity to assist in and around crime scenes while they are being processed. This will improve your chances of becoming a CSI while adding to your knowledge.
Regardless of how you get there, once you are selected to be a CSI, you will likely receive advanced training at an academy and serve a period as an apprentice before becoming a full fledged crime scene investigator.