What is the law on how long I can wait to press assault charges on someone?

I'm a canteen manager. I had to go a write up on an employee. He first refused to sign the write up. I'm new at my position as canteen manager. He got really mad and spit in my face. I told him to clock out and leave. He did. My boss told me to write him up again for that. He has been off work out of town. I haven't seen him in almost a week. I'm not sure if anything is going to be done regarding what he did, so if they don't, I personally want to do something on my behalf. I'm disgusted.