How long should my sister be grounded?

[Irishfan]

I’m the second oldest child out of 3 kids. My sister is 21( commute to college), I’m 19( commute to college), and my younger sister is 15. My parents are in LA for their 25th anniversary. They’ll be there a few days, then they’re going on a cruise.( They’re using all my dad’s vacation days in 1 swipe). Ally, the 15 year old, is just so difficult to deal with. I don’t even know why she came to me with this. I was watching TV when she approached me.

Her: Hey, I’m just going to cut right to the chase. My boyfriend is coming over tonight. Will you buy us beer?
Me: What are you asking me for? I’m only 19, Michelle( the 21yo) is the legal one. Let’s ask her. ( I call Michelle down and tell her that Ally wants beer)
Michelle: Absolutely not. You’re far too young.
Ally: C’mon please, just this once, I’ll never ask you again. I’ll make you a deal. We buy a 6 pack, you and ( my name) take one, me and ( her boyfriend) take 1 and you two drink the other two whenever you want.
Michelle: No deal. Drop this now. Knowing you, you’ll keep asking in the future and you’ll probably put it on Facebook or something.
Ally: No, I’ll never do it again, no Facebook. Please.
Me: Ally, let it go. It’s fine.
Ally: F**k you both.
Me: Alright, Loverboy definitely isn’t coming over, give me that phone and then go to your room. You’re grounded.

I forgot to tell her how long. How long should we ground her fir ?