For about four years, my group only played every six weeks to two months. It worked fine. We played all weekend -- probably 8-10 hours, and I set up the adventures so that the party leveled every 2 sessions.
A few tips:
1) Schedule early. I would usually send my group an email immediately after we played asking when folks could play again. Give your group plenty of time to plan for the game.
2) Be flexible. Be prepared to change the dates you're going to play. Sooner or later somebody will have a conflict, and you'll need to reschedule.
3) Plan your adventures around the time you have. Ideally, the action should start, progress and conclude in the course of a single session. You can run a campaign with a story arc, but every session should be a discrete part of the arc. Try to avoid stopping in the middle of things.
4) Manipulate experience points if necessary. Some folks get bored if it takes 6 months to gain a level. I get bored DM'ing the same level characters for too long. If your group feels the same way, pick up the pace.
5) Know what the adventure is before you show up to play. Do not spend the first two hours of the session dropping adventure hooks and watching the players look for the dungeon. Drop plot hooks at the end of the session, and let the players decide what they want to do during the time between sessions.
6) Use the Internet. Use mailing lists, blogs, wikis, messageboards, whatever. Use it to schedule your games. Use it to decide what the players' are going to do next session. Do the players have a choice between going north to the haunted castle and going south to the ancient pyramid? Put it in an email and find out what they want to do so you can be ready to play when the time comes. Do the players need to talk to the king? Consider an in-character message board. Definitely handle experience over the net. If you let PC's buy and sell magic, definitely handle that over the net.
7) If distance is an issue, consider a Virtual Table Top. I hated the idea of playing D&D on a computer until I tried it. Using a VTT does not feel like a video game or an MMORPG. I play a lot more often and drive a lot less now since I started using a VTT.
Good luck!