When dating online, when is waiting to meet someone too long?

I have been talking to someone for 6 months through email, instant messeger and the occasional phone call. We live roughly an hour and half away from each other and both have children. But I have made the time to get together and when it comes down to the day it either gets canceled or something happens where we don’t meet. I am attracted to this person when it comes to the phone and internet but is it real or is it just an illusion? I have tried dating others but I can’t seem to get the wonder of this person out of my head. When does it just become too long to talk to someone you like online when you know you are making ever effort to meet them? When do you just let yourself move on and forget them all together?