IMO, stop by and check with him. It’s always best practice to get the other person’s OK so they know to maybe expect a call. I don’t know that there is a time limit, as long as they can reasonably and truthfully vouch for you. 3 Semesters isn’t that long ago, really. I have a reference from one of my ex-bosses from 10 years ago that I still use. I always call her to let her know I used her again.