Sure the cycles can be shorter. I would recommend at least 8 weeks though, but if you can't fit 8 weeks in to your schedule, cut it down to whatever you can do. Cycles can be longer too. You can pretty much keep your cycle going until RBE sets in, you are burning out or your joints can't handle the heavy weight any longer.
As for SD... I would go at least 9 days. If you have a longer cycle, or if you didn't do 15s this cycle, I would take a longer SD. Last time, I about 2 1/2 weeks off for SD and when I started over, I really felt it. Incidentally, this cycle seems to be turning out to be my most productive to date, though it is kind of too early to know for sure.