Forensic pathologist Dr Marjorie Turner told a jury that "textbook evidence" suggested the teenager would have passed out after about 10 or 15 seconds.
During that time she might have scratched her own neck, trying to claw at the ligature, but the person who strangled her would have to hold on for another four or five minutes to bring about her death.
The High Court in Edinburgh also heard that it was a mystery why 16- year-old Ms Doyle did not appear to have fought her assailant at any time before the ligature tightened across her throat.
The teenager's naked body was found just yards from her home in Ardgowan Street, Greenock, on the morning of June 2, 1986.
John Docherty, 49, now of Hunters' Quay Holiday Village, Dunoon, denies murder. The trial continues.