Mestinon should take effect soon after taking it. I would usually notice it within twenty minutes, with full effect within an hour. It is short acting, so clears out fairly quickly.
In my case, and remember everyone is different, I could not tolerate a full sixty mg of mestinon. The effects were too pronounced for my comfort and ran out before I was scheduled for the next dose. So I had wide swings of strength and absolute weakness. My solution was to take thirty mg every two hours instead of sixty every four. This works fabulously for me as I have a constant comfortable level at all times.
My doctors do not have a problem with this as long as I do not exceed the prescribed dose.
I have this same problem with other meds, where I absorb the maximum dose immediately, and am left without the needed effect until time for next dose.,