...
My rule of thumb is to send an email 10 minutes after the scheduled time to meet, and I drop the call 15 minutes after the scheduled time to meet. In the note I say, "Hey just wanted to see if you're still available to chat; I'll hang around for another 5 minutes". And a lot of times I'll get an email 30 minutes later saying, "Sorry, I forgot!" and other times I'll get an a response immediately because they just forgot about it.