Go here to read an interview about the software used on the Mars lander: http://news.oreilly.com/2008/07/the-software-behind-the-mars-p.html
"In cruise, we had contact about every three days for eight hours. During EDL, we had continuous contact for about I don't know 24-48 hours. Maybe it was longer than that. I'm not really sure. On the surface, we get at least two passes, two full communications passes a day. We actually get passes about every two hours during nighttime operations. The spacecraft has to wake up every couple of hours and verify its power state, and sometimes there's a communications pass associated with that. Generally in terms of command ability and getting science data back, we do two or three passes a day."