This is just me but I've had cats for years, have 5 of them (indoor only) and even when one of them wasn't diabetic, I'd never leave them home alone longer than overnight, but I'm a worrywart.
The concerns might be that kitty is so stressed out to be completely alone for that length of time that she could stop eating altogether and that puts her at great risk for health issues, name Fatty Liver Disease (which can be very serious and very costly to diagnose and treat). What if for some reason your fountain stopped working?
A lot of people will hire a pet sitter, there are many pet-sitting companies where the people are bonded, insured, it's a business.....and they come into your home once or twice a day (once should be enough) and ensure your pet is okay, ensure fresh water, ensure food, scoop the litterbox, etc. It's generally pretty affordable. I've used these services many times in the past. You meet with them first in your home...they meet your pet, and you, and you ensure they're someone you have a good vibe with......confirm what services they provide and cost....and if you feel comfortable you provide them with a key, your contact info, ensure you let them know who your kitty's Vet is, in the event that kitty needs to see a Vet (eg: becomes injured, stops eating, etc)........ This is a lot better than boarding a pet because the pet can remain in their own familiar environment.
You could call up local Vet clinics and ask them if they know of any pet-sitting companies they'd recommend; sometimes you might even find that some of the staff do this on the side for extra money (like a vet assistant, vet tech, etc).
how old is your cat?