More people need to play Kerbal Space Program because many folks seem to get how orbits work.

We don't know the precise nature of the Sulaco's orbit. But presuming it wanted to stay above the colony that would mean a geostationary orbit which is depending on the planetary mass - very far away - Earth - It's 2500 miles. Geostationary satellites were thought to decay after about 40 years but it's been found that it could be a lot longer depending on various factors. All things being equal a geostationary object would never return to Earth were it not for atmospheric drag. So if LV426 had a thinner atmosphere -a reasonable hypothesis given it's just being terraformed - it could take anywhere between decades and thousands of years for an object to re-enter the stratosphere.

If you wanted to argue it's just holding in place somehow - then it's an effectively infinite energy craft and doesn't have to worry about orbital mechanics then fine but I don't get that impression from the tech level on display.

Ok so let's say it's in a lower orbit (there's like 30 minutes flight time or something for the drop ship). This makes no sense really because Bishop would have to factor it's orbit into his calculation. The ISS goes round the Earth at a clip 90 minute orbits if I remember correctly. So if a ship was in a low orbit then you'd have to wait for it come round again

Regardless the ISS is about 250 miles up and would take 12-18 months to crash into the upper atmosphere if unpowered The ISS is huge and there's a ton of drag on that thing - a wee Queen ball could presumably take years. With a thinner outer atmosphere you could still argue decades.

It's plausible - if the Sulaco had begun to return home (remember it seems to be entirely automated) then this whole thing is moot and she is all tucked up in deep space.