Like two minutes?
The only realistic means that modern earth has of stopping even a single Astartes in combat is a tactical nuclear strike.
Any tech marine (likely any marine in their armor actually) would have the internet and the entire global telecommunications grid under their control in basically seconds.
Realistically speaking, a Battle barge showing up in orbit, performing a demonstration lance strike, and then demanding the planets surrender over the entire telecommunications grid would have Earth surrendered basically just as fast as the situation could be processed.
No, I disagree.
It really comes down to how far the Space Marines are willing to go to take the planet. Using nuclear level firepower basically destroys the whole point of conquering a planet. The same reason we generally don't see other nations just blasting another nation into oblivion; the general goal is to obtain the people and economies, so nuking them is just pointless.
The Astartes lack the numbers and I would argue, the military sophistication to really take the planet. And threatening the entire planet at once is likely to ensure that you could potentially face not only tens of millions of troops, but not-inconsiderable nuclear arsenals. And there's really no way to ensure that they don't build more powerful nukes to blast through the shields.
I also sincerely doubt that a tech marine or even an entire chapter could really gain control of the internet without a great deal of struggle. First, we already know that a couple of nations like China, Egypt, and Russia have kill switches to the outside world. So trying to take over their internet is going to be entirely pointless. Second, a great deal of information can be cut off the internet, so it's not really going to serve them long-term, since the Earth powers would just change their strategy.
I think that, given their low numbers, the Astartes would be better off attacking a single nation and avoiding a large confrontation. Given the large technological disparity, they should choose a country that has great defensive geography, but is not terribly sophisticated technologically or has a good number of allies. My personal choice would be Iran; not many allies, the few allies it has been at odds with other allies, not terribly sophisticated technologically, lots of institutions that can be converted to the Astartes cause, a very religious society, and is geographically difficult to invade for other powers.
Doing this allows the Astartes the option of joining the table as one of the players on the planet, not as a threat that the whole world must face. This is a strategy that will take decades, if not a century or two, but has far greater chance of success to obtain what the Astartes actually want; people and their economies. The Astartes can coast on their technological advantages for some time at least. Iran has about 84 million people, which is a nice clean scoop of a population to put in the hands of the Astartes.
Best of all, the region around them is vacant of any real major power, save for Turkey to the north and a minor, but heavy punching Israel to the West. The Astartes can forge a new take on Islam with the local population, by essentially saying that the Emperor and Muhamad are the same. The warrior culture of the Astartes will do very well here and their advanced technology will give Iran a new lease on its pride, which basically wallows in the mud.
I still don't think that the Astartes can really dominate the world, because all the other powers are going to be struggling to obtain as many of their secrets as possible and I think they're likely to succeed. That said, the Astartes could probably take the former territory of the Ottoman Empire and become a lasting great power in the region. Maybe move into the declining Russian space and capture large swaths of land, but I expect that doing so would immediately draw action from Europe and the United States.
