The hardest wait is waiting for someone. Expecting someone else's attitude can cause suffering.
I'll give you a personal example: I broke up with a great friend, both were wrong in the story, she hurt me and I hurt her too, but before we left for good, I invited her to come to my city; I would pay for her plane ticket, she would stay at my house, in short, everything would be resolved if she came. But she said no, she said ''I can't'' But she could come if she wanted to, I know that because I know her and I know her life. Well, it's been 4 months now, 4 months we haven't talked or seen each other on social media because I excluded her from everything. I miss her, but what can I do but wait? She said ''I can't go'' When she could, when I did my best to try to fix things by calling her to come here. I don't know if she will ever really come back and we can mend our friendship of more than ten years, but I feel like I shouldn't follow her, go until her, I feel like I should wait... But how long to wait? Is it worth waiting for her? These are questions that still cause me some kind of pain, although this pain is less and less. But it's like that little pain of waiting was the only that still keeps me connected to her.
I agree with you that it's harder to wait for someone that is why we choose for ourselves what or who we wish to wait for but the important thing is, is the person or something worth waiting for? Don't wait time in waiting for what or who wouldn't come by.
As for you and your friend, if you value the relationship very much, keep in touch with her and let her know that you're waiting. Don't stop waiting if you think she's worth any length of your wait.
Just my suggestion though.
I can understand how you feel about waiting for her.... The hardest part is that she said she will come when she wants to.... Maybe you should do others things just as @merit.ahama said while waiting for her, that way, you do not start taking note of the waiting period.
You are lucky she said she will come, someone I know told me, I shouldn't even wait..... Even when I want so much to wait.... I have to do what that person wants as long as it makes him happy