I said before 6 but partly thats because our alarm goes off at 545 during the week for work/daycare. Occasionally he will sleep until 630 but not often.
I'm glad to know my kids aren't the only ones with early biological clocks. My MIL once asked why our kids go to bed so early (7:30 for Caleb and now 8:30 for Abby). She didn't understand that if I let them stay up later, they're still not going to sleep in.