He has full control over his arms (and honestly any of his body parts) even when unattached from the rest of his body. You could probably chop him to bits and he'd still theoretically be able to function, though his coordination would definitely suffer and it would take a lot of magic. The more separated parts he has to control, the more sloppy it gets. Two separated arms would probably be his reasonable limit (and sometimes they are both separated).

Both of his arms are usually sutured into place and they're hard to remove accidentally. To make them float around, he'd have to either cut the stitches (as he's likely done here) or get some help ripping it off (same effect but more damage to his actual tissue). The medics hate him because he does this so much that he's constantly in to get his arms sewn back on.

He can control his arms pretty effectively at a great distance, but he has to know where an arm is to be able to control it. If he were asleep and someone ran off with his arm, he might be able to fidget it a little bit, but he would not be able to fully control it because he has no concept of where it's located and where it is relative to him.

His coordination is also somewhat based on how well/whether he can see his arm. If he moves it behind an obstacle, like a bookshelf, he's way more likely to knock a bunch of books over or hit someone or something because he can't see what he's doing. He has similar problems if his arm is fairly far away, distant enough that he can't see it clearly.

If his arm ends up far away, he's usually able to just reign it back in closer to his body, as long as he knows where it is (and, of course, it's easier if he can see it). He might have to walk over and pick it up if he's lost track of it somehow (or if his magic was interrupted - usually there are just passive magic tethers tying his separated arms to the rest of him, but those can be broken under some circumstances).
Sometimes he just traipses over and picks up his arm off the floor for comedic effect, as well (usually if someone has grabbed it and thrown it, I imagine people around him are tempted to do that a lot!).

Now that I have all that written out, I realize it's a little hard to explain all the nuances of Nico's floating arms!