I’m not engaged yet, but my gf and I plan on it in the next few months, so I’ve gotten my fair share of advice on the subject.
My advice is to pray and make sure that both of you are on the same page in every way about finances, children, NFP, where you want to live, etc. If you’ve been dating that long, I’m sure you’ve already exhausted these issues, but it doesn’t hurt to rehash them before tying the knot.
Once you both believe that you are being called to marry each other and you have a good outline for how you see your lives together going, I say get a ring. Unless you have financial or scholastic commitments holding you down.
God bless the two of you!