I am on a 12 month lease which allows for three unrelated persons to live within the house, my girlfriend stays the night quite often and hangs out a lot, I was simply wondering when this becomes a legal issue with my lease, I heard three consecutive nights but I'm not sure if that's correct. I'd be grateful I'd you can help me out.

That would depend on the terms of your lease. If no specific time is defined, you may want to look at what your state defines a short term rental. For example, it may say that a short term rental is 14-days or less. This definition is used for hotel/motel/condo-hotel operators get licensed (or registered)for operation. With no other reference, that would be a good start.

Thank you I'll look into that, as there is nothing in my lease about long term visitors

An argument about who is and is not a resident involves where the person gets mail, that person's legal address, where clothing/makeup are kept.

I don't understand how a short-term rental is involved unless the friend is paying rent.

Her legal address is not at my house and she won e getting any mail, but she might leave some clothes and things around. And she would not be paying rent, she has her own place, my house is just nicer. I'm mostly concerned about this because my roommate was complaining.

I think your roommate could have a legal argument - she leaves clothes and things around? I think if the roommate is unhappy enough he could cause a problem concerning residence. You could argue it's not her PRIMARY RESIDENCE. He will argue that she stays over frequently enough to consider her a resident.

I can understand both sides of this. If she is using utilities (including water, electricity, whatever else is invovled, watching cable) he thinks she should pay her share.