When should I let go?

Situations like these are hard. There's no signal or hint that the person will come back or not and waiting is just too hard on the soul. In situations like this there's really only 2 options, wait or move on. If your friend won't answer you when you speak to her or they're just completely ignoring you after you've given them time, maybe a week or two. Then it's not worth traumatizing your heart over it. Give them a set amount of time and after that move on. You can try talking to then after that given amount of time to say I respect your space but I'm still here if you need me. And if no response then move on. Worse thing that can happen is you move on and she comes back.