My recollection from the extensive testimony about bugs in the Danielle van Dam 2002/2003 murder trial is that blowflies can help narrow down TOD to within days, and maybe to within a day, but in this case we're really talking about a max timespan of probably 6 to 8 hours (i.e. was Nancy killed between midnight and 7am or sometime after 7am?). I doubt any bug evidence will be able to tell them with that degree of preciseness. And of course it depends on where her body was and was it 'stored' somewhere, etc.
I think everyone pretty much agrees with the fact that she was dead at some point on Saturday morning, and thus was not held captive somewhere and then killed Sunday or Monday.
Other measure are used to determine TOD (rigor mortis, liver temp) but those are used when the death has occurred and the body found within 24 hrs. The level of decomp on Nancy may well have precluded any of that information being possible to gather.