If a soldier was sexually abused in the military how long can this issue be investigated?

If you or someone else is reporting a crime you should contact the previous command/base and local law enforcement for assistance. They are the subject matter experts and will be able to assist you with the details to include statute of limitations. You can also report this to the VA to get treatment for MST. I wish I could do more but with the limited information that is the best I can recommend at the moment. Good luck.