Time isn't the deciding factor, the purpose is the deciding factor. Grave robbers dig up graves and loot structures to obtain artifacts to legally or illegally sell (depending on the host country's antiquities laws, most disallow looting). The only thing grave robbers are interested in is making money. That's it. They have little to no interest in actually learning about the culture or people that made the item. They have no interest in understanding how humans adapted to that region, lived their lives, and influenced other people around them. Their only interest is money.

Archaeologists, on the other hand, are interested in answering research questions that hopefully benefit humanity by providing a better understanding of our past and where we came from. Yes, archaeologists dig up graves and study the artifacts, but they do so to answer questions. Questions like, where was this item made? How was it made? Is it decorated and if so, do the decorations mean anything? Is the quantity of material recovered in a grave significant? Did the person have great status alive or was their status through grave goods one given by others upon their death? And so forth. Under no circumstance are archaeologists selling the artifacts they discover. If they are, and are caught, they go to jail and pay heavy fines. Any material recovered either goes to a local university or museum within that country or to a storage area for future research and analysis.

That means if an archaeologist has a research question that is only answerable by examining remains from a few decades ago, they must go out and examine those remains. But then, an archaeologist doesn't necessarily need remains to answer research questions. They can examine the remains of agricultural infrastructure, buildings, temples, the landscape itself, lake sediments, middens, artifacts, and so forth.

In fact, a group of archaeologists did a research project aimed at understanding homelessness and what homeless people may need that they are not getting (Zimmerman et al 2010). By taking an archaeological approach to understanding the material culture left at homeless encampments as young as a few weeks, researchers determined that Bibles and soap were the least needed things given by aid groups to the homeless. Bibles were useless because they could not be eaten and soap was useless because there was no water in which to use it. What the researchers did find was numerous exploded cans of food caused from heating the cans without opening them. What the homeless needed, instead, were can openers. They had all this canned food given to them to eat and the ability to heat the food with a fire, but no effective way to open the cans.

I would like to note that there is an entire field called forensic anthropology which deals with understanding death, how people die, and how they were killed usually in regards to a crime. Out of necessity, forensic anthropologists dig up and work with human remains that can be quite fresh.

So as you can see, there is a difference between grave robbing and archaeology. And while that difference may not seem significant to some, it makes a world of difference.

• Zimmerman, Larry J., Courtney Singleton, and Jessica Welch. "Activism and creating a translational archaeology of homelessness." World archaeology 42.3 (2010): 443-454.

Edit: As ThesaurusRex84 said, archaeologists need the permission of the landowner to excavate. They also need permission from any local, state/province/territory, and country/federal along with any necessary permits. You cannot just go in and start digging wherever and whenever you want. Grave robbers, on the other hand, don't always seek permission. If they do, it's usually to give the landowners a cut of the profits or objects found.

Another difference is documentation and publications. Archaeologists document everything about their work from specific location of the site, where excavation units were placed, what objects were found, the frequency of objects, any remains recovered, etc. They then write any necessary reports to the levels of government that require them. After that, archaeologists will publish some or all of their findings in a peer-reviewed source so that others can weigh in and judge their work. Grave robbers don't submit reports and don't publish their findings. They may document where they robbed, but it's only for future robbing.

In Europe archaeologists tend to be employed throgh the state, either directly or as subcontracted businesses. And most excavations are 'emergency excavations': they are conducted because that plot of land or area is about to be developed or dug up for some reason. So here time is an important factor in terms of running out of it. Typically archaeology here is anything more than 100 years old (that's not a generalisation, that's usually the rule). Of course, sometimes that is bent a little (for World War Two sites, for example).

In the UK, the onus is on the contractor to fund the investigation or excavation to ensure they don't destroy something important. In the countryside, this means a quick look at local records to make sure there was nothing there, rarely resulting in excavation. In London this means excavating, because odds are you will come across a either a grave site (medieval, most likely) or some Roman stuff (see the Museum of London's publications for the sheer number of inner city Roman excavations), and these is also likely to undocumented. There are exceptions - the Spital Crypt Project was the removal of hundreds of coffins from a decaying 19th century crypt in order to make much-needed space for the living (there's a link below). In this instance, the remains were carefully removed and examined by archaeologists and professionals. The aim was to asses the remains in ignorance of known data (most of the coffins had name plates, with dates and ages on them) and then compare their analysis with the the facts. It led to a re-assessment of how remains are aged, for example, as most of the team did not believe that people from that era typically lived to an advanced age. When contrasted with the documentation proving that they did, this meant that they were habitually under-estimating the age of the person at death. So assesment and analysis practice was changed as a result, which leads to re-assessment of other remains and ultimately to new knowledge that may affect how 'modern' human remains are assessed forensically.

In Belgium, investigation and excavation or almost a standard, due to World War One and World War Two - there is a very high likelyhood of coming across human remains, UXO's and the like, even in rural areas. In this instance, careful excavation removed the threat of people exploding, while simultaneously transferring the dead to a consecrated grave with due ceremony. One fo these sites is Pheasant Wood, where remains from a mass grave were relocated to consecrated ground. Another is Boezinge, where excavations revealed the remains of over a hundred soldiers in an small area of what had been No Mans Land in 1916. At Boezinge, amatuer archaeologists were actually excavating because there weren't enough professional archaeologists to go around - the site was undeveloped and believed to have nothing of interest as nothing was found on during the intitial documentary research.

Excavations that are planned in advance and that are not driven by immediate need are usually for big projects that aim to learn something; it will tell us, now, about them, then. It is only justified if it a) contributes or expands our knowledge or b) is an emergency. Needless to say human remains are carefully excavated, with reverence and attention. Rural excavations are not conducted with the expectation of finding human remains, and doing so results in calling the police, just in case. After the police determine that it's ancient, then a permit must be obtained to continue.

There are sites that are of limits: these include war graves, such as sunken warships from World War One (in the UK, even diving around one is a dubious) and most known graveyards. If a known graveyard (that is, documented and consecrated) was to be excavated, it would be as an emergency, or with express permission and direct invitation from secular and religious authorities.

So to conclude: ancient graves are usually discovered in awkward places, and end up being excavated because people want to build there or because it's at risk of being robbed; in so doing a lot can be learned about the people of that time/place. Modern-ish graved are protected becuse they are contained within a recognised and demarked area. The more recent they are, the higher the likelyhood that we know quite alot about them, usually through documentation. We don't need to dig them up, either for knowledge or for practicality.

There are archaeologists who do not excavate human remains, and refuse to do so; there are achaeologists who specialise in human remains, and those in the ethics of excavating human remains. There is a code of conduct for excavating human remains. The gist of it is simple; do we need to do this? Why? If yes, then continue with respect and empathy.

References (by no means perfect): http://archaeologydataservice.ac.uk/archives/view/spitalfields_var_2001/reports.cfm?CFID=426939&CFTOKEN=ED19A63B-BCB2-4409-A6DAB49409CCB923.

http://www.mola.org.uk/publications-all?location=62&period=58&topic=All (SCROLL DOWN)

Protection of Military Remains Act 1968

http://www.mausershooters.org/diggers/E/activiteiten/yorkshire-trench/restauratie.htm

https://en.wikipedia.org/wiki/Fromelles_(Pheasant_Wood)_Military_Cemetery (sorry, I couldn't find a non-wiki link)