Threats never spacewalk. That's a solitary comfort when you watch the part of the ship that's holding the hungry alien or the murderous ghosts just go drifting off into the great unknown, ain't it?
There's no maximum distance for a spacewalk. It's sufficiently high risk and high reward that so long as you can draw a straight line between your module and your destination, you're golden. Just...y'know, don't die. Simple as, right?