I have a fun hypothetical for you to chew on. It starts like this, when does the 2 hour timer start? At the start of the morph? The end? The middle? (Theres also the question of why it exists but thats another post.) But I presume the end for reasons like the wolf bit early on and the various other times they cut it close. But consider this. (We're ignoring for plot/having risk in the series reasons.) What if you can hypothetically reset the timer? By starting the demorph just enough to Feel a change, then going back the other way. If the timer only starts on completion of the morph. Youve completed a morph. Theortically, one could spend all day as a bird, then go home, so long as they kept restarting the timer.
I, too, have wondered about this so much. Like, if you're Cassie levels of talented, could you stay morphed as a dolphin and then only turn one fin back into a hand at two-hour intervals and just... keep on swimming all day? We don't get any answers — probably because our protagonists are kids and/or humans who don't know the science. It's probably for the best that they err on the side of caution.
But if I had to guess, it'd be "you have to return completely to your base shape within ~120 Earth minutes of starting to turn into another shape, or else you'll be stuck." Mostly from #3, when the four human Animorphs struggle to demorph from wolf after 2+ hours in morph. They all have to fight to get the morph started, and then also have to fight hard to keep it going. It's to the point where Tobias thinks Marco's going to end up stuck as a half-wolf half-human hybrid, unable to morph any further in either direction. That says to me that the only way they can avoid getting stuck is to go entirely back to human shape before the time runs out.
As to why the 2-hour limit exists... I've speculated about this before, but my theory goes like this:
Your original (e.g. human) mass gets schlooped into z-space through a singularity when you morph (#45).
Your human mind remains connected to your morphed (e.g. hawk) shape, even though the literal "brain" that's sending out the mind is now elsewhere (#18).
That connection decays gradually over the course of the first ~115 minutes in morph, and then very quickly over the next ~15 minutes, until it's gone. Sort of like a car that needs minor occasional repairs for the first 200,000 miles, and then has a dozen parts break at once between 200,000 and 250,000 miles.
Once the connection has decayed completely, the human mass is no longer able to pass through the singularity. The human mind remains intact, the human body is still suspended in z-space, but the hawk shape can't be reversed because the connection has broken down.
Mostly that interpretation comes from the close calls like in #3 and #21. It's apparently still possible to demorph after the 120-minute 0-second point, but it's excruciatingly difficult to the point where highly talented morphers like Cassie struggle and less-talented morphers like Marco can barely make it happen. And once you're even a few minutes outside the "difficult" zone, "difficult" becomes "impossible" (#1, #22).
