There's no need to write each other for weeks, even though you may have grown accustomed to that nice warm shot of adrenaline you get when they've sent a new message. As much as you can learn about someone over the phone or computer, you need to find out if the two of you have that oh-so-important, never-underestimate-it, all-hail-the-elixir-of-love chemistry.
Four to five back-and-forths on email will give you a good gauge of character. You can learn a bit about them but will still have many things to uncover in person. And if they can't even commit to that teeny amount of contact, don't pursue.