Both my girls were formula fed and I found that until they were about 3 months old, there wasn't that much of a consistent time between feeds. I would suggest demand feeding but with a goal of sticking to EASY. What I mean is, stick to the EASY order of things but pay attention to her cues rather than watch the clock. At 6 weeks, it's pretty hard to get them to stick to times. Once she gets about another month older, she will naturally move into more regular timing.
You can always keep journalling her routine to see what she is doing. In another month, I would them think about trying to get her into more regular times.
Hope this helps! At 6 weeks, we were just going with the flow and trying to survive the sleep deprivation!
Good luck,
lilysmommie