how many years should you stay BF/GF before getting engaged.

I don't think marriage is something you should place a figure on. If you want to get engaged it should be based on love, not a length of time. So I think you can get engaged to the person you love when it feels right to you.

You know, normally I don't like to put time-restrictions on things like marriage, but in the case of an engagement lasting longer than 3 years, it's pretty much time to poop or get off the pot, as we say in the south. If you've been engaged long enough to be considered legally married according to your state's common law, it's probably been too long.