When is it too long to ask for a letter of recommendation from someone?

I’m applying for a college program in which I must include two letters of recommendation (which instead of letters it’s more or less basic paperwork to fill out). About 3 semesters ago I had a professor that I really clicked with, made a good impression on, etc. not to mention this professor knows how I perform academically.

Question is, is it too long to pop in and ask for something like this? I haven’t spoke to this person in months. Also, how exactly should I approach them? Just blatantly ask? Or is there some sort of tact/etiquette to follow?....