It really depends on where the researcher is. In the New World its between 75years to 15,000 years (100%) because there is little to no evidence for any humans being here before 15,000.

In Europe, the Middle East, and Africa its all over the board from 3.5 million and earlier in Africa to WWII in Europe.

In Asia it can also be all over the board but not as early because the best evidence for Homo Erectus in Asia starts at around 1.5mya.

There really is not a blanket answer to your question. In Europe it may depend on what is found during a pre-construction survey project and what each individual country finds important. In Italy they may bulldoze Roman era house sites just to excavate formative village or the Neanderthal site underneath.

In Kenya and Ethiopia they really focus on the early pre-human stuff, which btw are heavily supported by the oldest Christian churches there. Now in Nigeria they focus on the Bantu empires.
In Asia it depends on what the politicians there think is important, China really puts a lot into their early man studies and the major dynasty archaeology, but Japan is more focused on its Imperial Age, and in Cambodia or Vietnam they tend to focus more on the age of the Khmer Rulers.

As bad as he was Saddam Hussien was better than most dictators when it comes to archaeology and he did a lot to preserve Old Testament era sites and artifacts. These ISIS folks that are coming in have already declared that all of the Old Testament artifacts and sites are to be destroyed like the Taliban did with the ancient Buddhist stuff in Afghanistan.