I usually get back in touch with them about a week and a half before their arrival. At this point, I let them know that I'll be sending them a map within the next few days via the coded email address airbnb assigns to them (my place can be hard to find), confirm their arrival time, and maybe remind them of some of the things in my listing description they may have failed to read. I like to send this about 8-9 days before, as guests may be busy, distracted, may be camping or otherwise out of cell phone or wifi range, and this gives them time to reply before I start getting anxious because they're not responding. Then about 4 days before their check-in, I'll write again to ask if they received the map if they haven't already acknowledged that, suggest useful items they might want to bring (flashlight, hat for walking in the hot sun, etc.) give them bus info, and so on.
Some hosts may not want to bother with this much communication, but because of where I live, both my guests and I find it is helpful.