Monster attack that must be sustained

I'm putting together a facehugger-like monster that will latch onto the back of the neck and burrow into the brain.

In a game with combat rounds I'd say something like "must stay latched on for three rounds to kill the target". How might I run this in MotW?

Having thought about it, maybe I'll just not worry about it :-)

Here's my monster, any opinions?

Mind-Eater (Kai’Jedlek)
Type: Devourer/Breeder

Out of Host
Description: A fat squishy blob about 6 inches long, with sharp claws and teeth
Powers:
•   Devours brain to control host (see Custom Moves below)
•   Scuttles short distances quickly
•   Leap up to 15ft in any direction (including into the air)
•   Can remain dormant indefinitely.
Attacks:    Bite/Scratch: 0-harm hand
Armour:   2-Armour from being a difficult target to hit
Harm Capacity:  4-Harm
Weakness: Attempting to devour a host with extreme levels of caffeine in the bloodstream will temporarily stun the monster – it falls off and remains still for about a minute. Armour is temporarily reduced to zero.
Custom Moves:
•   Devour Brain: Latches onto the back of the neck and burrows into the host’s brain through a small puncture wound, effectively killing them and taking control of the body.

In Host
Description: To a casual observer, the host will appear unchanged. More careful observation will reveal a small wound at the back of the neck, slightly clumsy movements and possibly slurred speech
Powers:
•   Remain in host for around 24 hours (between 12-36 hours depending on body’s condition)
•   Full physical control of host (with reduced fine motor control)
•   Access to memories of host
•   Exceptional physical strength
•   Ignores pain
•   Reproduce single offspring in current host’s abdomen approximately every 3 days

Attacks   :
•   Punch: 2-harm hand
•   Improvised weapon: 3-harm hand
Armour: 1-Armour from being insensitive to pain
Harm Capacity: 8-Harm
Weakness: If the host dies in the first hour, the Mind-Eater has not matured enough to escape and will also die
Custom Moves:
•   Escape Host: Bursts out of the back of the neck and reverts to “out of host” stage. (must have been embedded in host for at least 1 hour)


(note: edited slightly, I missed a few things)

That looks good. For the time taken to burrow in, you can model that by the usual "say something, then ask what they do" pattern:

"The thing leaps on to the back of Frank's neck, what do you do?"

"Frank, you can feel it burrowing into the back of your skull. What do you do?"

"Now it's completely inside Frank's head. What do you do?"

When it comes to AW games, here is how I handle every "how does this work" question: If I was watching a TV show/movie, how would it work on screen? For MoTW, I always default to Supernatural and/or Buffy.

So, how would a facehugger creature look on one of those shows? Would there be an epic struggle? Would the hero's have to back away quickly?

I know it doesn't exactly match your description, but a couple of years ago I needed to design an alien race and came across this artist's image, which I thought was a fantastic parasite-y thingy. Fits the concept well: