I tend to give it two or three times and then let the ball remain in their court. But I'd also add that it's often worthwhile--especially with old and/or good friends--to simply talk about it with them. Don't assume anything going into the conversation and don't be confrontational. Just simply tell them "I miss you, and my feelings get hurt when you don't respond to my invitations to get together. Is there something wrong." Hopefully, that might prompt a dialogue that will, in the very least, let you know where things stand in the friendship. Maybe you'll learn that something's been going on in your friend's life that has hindered her ability or desire to reach out. Or maybe you'll get the sense that your friendship, sadly, is waning.
Regardless of their response, at least you've articulated your feelings about it. If there's one regret I have with friendships that have atrophied and died over time, it's that I didn't articulate my feelings and sadness about it. Because of that, I feel needlessly bitter and regretful about it. I might have saved either the friendship or my dignity if I'd found the courage to express my hurt and sadness.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher