Elfangor must know our language and how our time measurement units work because
he has already been living on Earth as a notlith in a human body.
Also, a second is defined as:
it is defined by taking the fixed numerical value of the caesium
frequency ∆νCs, the unperturbed ground-state hyperfine transition
frequency of the caesium-133 atom, to be 9192631770 when expressed in
the unit Hz, which is equal to s^−1 https://en.wikipedia.org/wiki/Second
So, as Andalites must be a civilization that has used atomic clocks (to have anything remotedly similar to the Global Positioning System in the Andalite homeworld at least). They must have also measured their minimal time units in something defined by something common in the universe, such as the vibration of some atom or something like that.
From there, you can calculate and convert the Andalite seconds to our seconds and then to our hours, and omitting some puny decimals.
Now, about the "being barely able to pass the time limit" it can be due to two things.
--The first may be thanks to those omitted decimals.
--The second may be because there has been a lot of simplification done during the development of the morphing technology to save computing power. You see, in engineering some graphics are simplified to make it easier to compute an/or leave some margin to prevent extreme failures.
In this example, the horizontal line is time and the vertical line is consciousness concentration in concentration-units (Invented just now). We can also say that only the Ellimist is able to reach past the 2 concentration-units mark and up to infinite.
This graph corresponds to the equation "y=e^(50x-104)".
Well, if we wanted to simplify this, we just say that y=0 until (2,0) and then y= ∞.
You see that if you reach two hours morphed, in the second graph you are a nothlit, because the amount of concentration needed to morph back is infinite, but in the first graph, if you get near the two hour mark, then you need a lot of concentration to morph back, and just at the two hour mark you may be able to change back but you don't want to get very close to that if you don't wish to be a notlith.