This specific scenario is something i've thought about a lot over the years, and even though it basically has never happened with me, I still think it's a really important discussion. Circumstances like these nuanced, and require a lot of context to really know what course of action to take, but it's a fair assumption that these situations are generally bad news. Ask yourself the following questions: where is she going specifically? How often does it occur? What type of friends is she going with? Is it her single hoe friends? Or married, loyal, and reserved women who dress modestly? Is it clubs and popular bars, or normal venues like restaurants? How far out of town is it? How late in the night are they staying out? Context matters.
In your case, I can't fully comment without knowing the context of your relationship, but given that you felt the need to make a topic on this I'm assuming that's because your gut is telling you it's bad news. Listen to your gut.
If you can determine it's bad news, you can ask your girl a simple question. Do you think your mother (or grandmother) went out with her single friends and left her husband for days to go out drinking, raving, clubbing, festivals, concerts, partying, etc? The mere idea of her sweet old mother/grandmother doing such a thing at any point is abhorrent - so why is it okay for her to do it? See how she reacts.
Here's the most nuanced part of this, and is purely my take, but I think i'm on to something. I think that setting boundaries by proactively "communicating" the terms and conditions of acceptable behavior can backfire if you do it too early. What I mean is, there's no value in telling your woman early on in the relationship what she can and can't do, as if to nip it in the bud and "negotiate" the terms. Why? Because if she IS the manipulative and sneaky type, you probably won't find out until it's too late, because you will encourage deception right off the bat and she may go behind your back and lie. I think the better course of action is to let women do what they want, and let them tell on themselves. Give them the impression of indifference or even naivete. Then, when they express their plans to do something like this, they'll think you won't care and can be manipulated. This is the time to strike. You tell them:
"I thought you valued our relationship. Doing something like this tells me you do not respect me or this relationship. You are free to do what you want, but just know that by doing this, I will reconsider the future of our relationship, and I will no longer agree to exclusivity with you" - or something to that effect, whatever makes sense in the context of your relationship.
A boundary has absolutely no meaning unless it comes with consequences. You cannot negotiate boundaries. Set them, and be completely willing to show her why they're not acceptable by walking away or whatever you need to do. Do not budge. Do not argue. Do not try to justify why your position is reasonable or not. Stand your ground, with a rock solid wall, and do not let her shaming or criticizing or other attempts to manipulate get in the way. Her reactions alone are enough to go off of. If she starts shaming, criticizing, acccusing you of being "insecure" or "jealous" or "controlling" WALK AWAY. If she genuinely communicates with you and seeks to understand why you feel that way, and agrees with you, great! But I think that's very unlikely unless she's simple naive or highly agreeable. Most likely, any girl that thinks it's okay to go out drinking for 4 nights with her friends (or anything like that) is bad news, and there's nothing you can do about it.
