Maximum carbon steer tube length?
In general manufacturers each give specific numbers for this, and it tends to be 30-55mm of total spacer between the headset and the stem. Beyond that there isn't really a universally agreed upon rule of thumb. I would use the number 30mm lacking any specific recommendations, including any large conical spacers that are part of the headset. (In other words, the big spacers or the headsets that use them as a top piece don't let you "cheat" here; what's in question is the distance between the upper headset bearing and the bottom of the stem). 30mm is conservative, but this really is a good area to do what the fork maker tells you.

The maximum steerer length for a given frame gets into the stack height of various stems and whether you're running a spacer above the stem (a good idea unless you're using a compression plug that reinforces the clamp area, in which case some manufacturers recommend against it), so it's probably not the most useful number. You need to know your stem's stack height to determine it. It's that number plus the stack height of your headset plus your headtube length plus the spacer limit plus about 3mm if you're using a 5mm spacer on top of the stem, or minus about 2mm if you're not.

It doesn't make a difference whether the fork is tapered or not.