How do you know?

How do you know when you're finished having babies? 
I'm pregnant with my third and I always told
Myself I wanted 4-5 kids, but this pregnancy is HORRIBLE! I'm not nearly as sick as I was with my first two, but being nauseated feels like this is worse than actually vomiting. I'm only 10 weeks and I'm ready to pop this baby out. 
I'm younger, so everyone says I'm to young to actually be finished, but honestly I don't think I want anymore babies after this one. 
I have two girls right now, my husband says if this is another girl, he wants one more baby to try for a boy.. But I told him that if this is a boy I'm done. 
Is it okay to say I'm done when I'm still fairly young? I just can't handle pregnancy anymore.