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What's the typical relation between a chapter and it's battle companies? Does each companie act independently, or are companies usually deployed in concert? How much time do companies spend away from their fortress/fleet and what kind of communication is required? (If the answer is "it varies from chapter to chapter", it would be nice to hear some examples.)

It depends on the chapter I think. The companies of chapters like the Raven Guard, Iron Hands and Space Wolves operate usually on their own.

Everything with Space Marines varies from Chapter to Chapter, if the Chapter in question is given a name by the studio. Named Chapters stand out for some reason or another, regardless of their Founding.

For any sort of long-range communication, though, Astropaths are required. This is true throughout the Imperium. Local, in-system comms can be handled by comm-lasers or vox-casters, but anything to talk to another star-system requires an Astropath.

The First Company is typically veterans and Terminators. Companies 2 through 5 are the "Battle Companies" and have a mix of Tactical, Assault and Devastator Marines. Companies 6 through 9 are "Reserve Companies", Companies 6 & 7 being all Tactical Marines, Company 8 being Assault Squads and Company 9 being Devastators. Company 10 is the Scout Company, and is almost entirely Scouts (save whatever Marines are responsible for their training).

So, in most cases, deploying a single Battle Company should be enough to execute most SM missions, and the Chapter can thusly deal with numerous missions at once.

The above assumes Codex-adherent Chapters. If the Chapter is not by-the-book, then none of the above may apply.

Battle companies are a chapters primary battle unit (hence the name) the Reserve companies are so named because they are typically broken up to bolster the Battle companies ranks rather than being deployed en-mass.

Astropaths are necessary for long range communication, Librarians are also periodically noted as being capable of functioning as short-range Astropaths.

The Marines Errant are an extreme example of operations, their crusade fleets can operate out of contact for decades at a time.