Why the alarms to eat and drink?
Kaylie learned from her research that some of the past owners of the mirror died from starvation and dehydration.
Why so often? (my personal interpretation)
Kaylie knows that the mirror is manipulative and the persons around it have delusions and lose control over time. She also believes that the mirror feeds on the life around it, so she experiments with a dog to prove it to her brother and we see that plants losing life and it also controls electricity.
Kaylie is being overprotective of herself and her brother. She believes that the mirror feeds on life (examples: experiment with dog, plants losing life). So, she might of thought it's best to eat and hydrate very often.
There's this review which you might find interesting. The reviewer says that all the safety measures Kaylie has taken are useless.
This mirror works primarily by messing with its victims' sensation of time and space. It makes you see things that aren't there, hear things that aren't real, and experience things differently from how they're actually unfolding. Sure, there's some sort of woman/demon/toothless ghost monster living in the mirror, but she doesn't seem to want to come out unless it is absolutely necessary. Normally, just messing with your perception does the trick. Which means there's a huge problem with Kaylie's "get over here and eat a granola bar" alarm strategy; it only works if Kaylie and Tim are lucid enough to hear it and understand what it means.