Even as a kid reading the books I always thought it was wierd that it was, at least, implied that they would eat by running but drink by standing still, so I just figured they absorbed at will since maybe grass you run on tastes better? If they can choose to make a distinction on when they'll eat and drink and such I figure it's up to them when they absorb.
But I sort of expect since they live off of grass that they would
have to absorb constantly or, you know, have an insanly slow metabolism, otherwise I figure they'd die what with all the limbs and with all the exhertion Ax and Elfangor are shown to be doing in the series and TAC.
But then I based that theory off of an episode of Gargoyles, my knowledge of metabolism as a child was indeed
that limited and hasn't gotten any better in my old age