It really doesn't make sense to talk about move-equivalent actions outside of initiative (combat is the most common case of explicitly using the initiative system, but it's far from the only one). Initiative is the system by which the timing of short-duration actions is measured (as compared to, say, a spell with a casting time of 10 minutes, setting/breaking camp, traveling across the plains, etc.), and the order of those actions is determined (does the orc get to swing before the rogue? roll initiative and find out! ... surprise rounds notwithstanding, of course).
Inside of initiative, the round is the smallest unit of time given an explicit measure (1 round = 6 seconds; no specific time is given for smaller actions within a round).
The question, then, isn't so much "how long does a move-equivalent action take" but "can I take actions while falling" and, if so, "how far do I fall in a round"?
For the first question, I can find nothing that precludes a falling character from attempting actions, and some explicit (albeit somewhat weak) support for allowing falling characters to take actions. Subjective Directional Gravity planes allow creatures to make a Wis check (as a free action, once per round) to set their "down" direction, and DMGII explicitly supports falling characters taking actions (immediate actions, to cast Feather Fall); it also speaks to the speed with which characters fall on, p47 (as pointed out here by KRyan):
A falling character reaches terminal velocity (roughly 130 miles per hour, or nearly 200 feet per second) within the first round of a long fall. That means a character falls roughly 670 feet in the first round. After that, a character falls about 1,150 feet each round. If a fall is from a great height and lasts several rounds, a character using feather fall needs to wait to cast the spell until impact is imminent, since the short duration might mean the spell expires before the character
touches down safely.
Thus, (a) the rules explicitly support characters taking at least some actions while falling and (b) 670 feet is the maximum height from which a character must fall in order to have a full round's worth of actions. Falling 670 feet would, per DMGII, allow you to draw a wand from a Handy Haversack and activate it, though your GM may impose penalties.
As I said, though, I can find nothing that precludes creatures from taking actions while falling, which would strongly support the idea that they can (characters typically can take actions, and conditions that preclude that explicitly say so; "falling" isn't a condition per-se, but it doesn't say that you can't take actions, so you should be able to).
So, can a creature acquire and activate a wand of Feather Fall while falling? Signs strongly point to "yes".
But, wands are slow
All of that having been said, note that, as written, wands
use the spell trigger activation method, so casting a spell from a wand is usually a standard action that doesn’t provoke attacks of opportunity. (If the spell being cast, however, has a longer casting time than 1 standard action, it takes that long to cast the spell from a wand.)
and, spell trigger says
Activating a spell trigger item is a standard action and does not provoke attacks of opportunity.
Nothing in the description of wands suggests that a wand of Feather Fall would allow the user to cast Feather Fall as an immediate action; rather, the wand would require a standard action to cast.
Therefore, to make use of a Wand of Feather Fall, the falling creature would need to fall at least 671 feet (670 feet of which are "used up" in the first round, the last foot of which is "feather-fallen" in the next round).
If you're that worried about it, I'd suggest getting a Ring of Feather Falling or figuring out how to fly.