I currently have a loop ileostomy (in between stages 2 and 3) so have quite a lot of output.
quite often I won't take any immodium, I do empty the bag fairly often, but I eat and drink absolutely everything, including coffees etc.
I am very active, do a lot of running and boxing so I think I would feel it if I was dehydrated but I seem to be fine.
however sometimes the output gets too liquid so I usually take one immodium a day and that thickens things up a lot. to the point that I start to get pancaking around the stoma so then I stop taking the immodium until the output is too liquid again.
this last 10 days I have been on a holiday where I did many day trips in the desert so didn't want to have to worry too much about emptying so I started to take one immodium in the morning and one at night. It really has made a big difference and I have been eating a lot more fibre out here and that seems to help avoid the pancaking.
I think for now though I will continue to take two a day because I do notice a difference at night, I seem to be able to sleep through the night much more frequently now without having to get up and empty