Pretty sure she was dead while still at Puma Path.
I doubt that any forensic technique will give a time of burial within hours or days. Can you say more about why the timing matters to you? She was missing already on Mother's Day (she was not alive, IMO). He's out running around Colorado.
She could have been still alive when taken to her burial site and that's a good question (tranquilizer dart theory). It horrifies me though.
She could have been moved several times (soil analysis should help with that). At the time of the murder, there were rumors about a particular area, near a Fire Station, where people saw Barry at some point. IMO.
Sadly, not sure an autopsy can tell if she was alive or not when she was buried. However, if anything like a gun was used in killing her, that will be future news. If instead he shot her with a tranquilizer dart, removed her from the Puma Path premises, and then killed her via some method such as smothering or biochemical means - we may never know.