Hopefully this helps...
My 4 mo old usually sleeps from 6 pm to between 6 and 7 am straight...this morning she slept until 7:30 am! Talk about lucky (we're waiting for it to run out). That was out fault though, we accidently slept in until then, we usually get her up at 7 regardless so she takes her naps at the right time. When she sleeps this late, sometimes she has a good nap day, other times she has a bad nap day. It's really like any other day. But it takes a load off my shoulders when she does sleep 12 hours, it means I only really have to make up for 3 hours in naps minimum, so if she has a bad nap day, she can get only an hour per nap and be relatively rested (although we've learned that our lo thrives best on 16-17 hours of sleep/day).
*HUGS*
Sonya =P