Well, everybody's limit of pet care is different, but I try to follow one general rule myself: never take any living thing into your care if you cannot provide for it in every sense for the ENTIRE duration of its life. That means an investment of time (attention, maintenance) AND money (food, vet bills, etc). I've seen WAY too many animals of every species that were bought/adopted on a whim and the owners just couldn't care for it one way or another... animals require long term thinking, even those with lifespans as short as a rat's.
If you're still living at home, you especially have to keep in mind what will happen to your charges when you eventually leave, for college or whatever. For example, I couldn't take my 10-gallon fish tank to college, and while my parents did *feed* the fish, I wound up with pea soup every time I came home
Wouldn't recommend parental pet care if they're anything like mine... but anyway, it's just something to keep in mind. It sounds like rats have fairly short life spans, but you would have to take into account the possibility that they'd be hitching a ride to college with you... and colleges are NOT pet friendly on the whole, trust me!
*steps off vet student soapbox*
just my two cents... like i said, i deal with homeless animals day in and day out, and my standard speech to any potential animal owner is just, research your animal and think long and hard about it! And should you decide on getting an animal, go for adoption!
good luck!