Glad to help rileyroo.
The organizations I've fostered for like you try to keep the animal for a specified time (3 weeks seems standard) but if it's a total disaster then they understand if you have to bring the animal back. Actually a lot animals "up for foster" are non-adoptable at the time, for health reasons (like my broken legged Husky), or because they are cruelty cases waiting for their ex-owner's court case to be over before they can legally be adopted. And since an animal often has a better chance of being adopted when it is located at the shelter, they won't expect you to foster indefinitely until someone is interested in the animal (unless you want to).
A good rescue will provide veterinary care through their own vet if they have one or through an outside vet where they cover the expenses. Everywhere I have fostered for also provided food, litter, leash, crate, etc for the animal. You should really only be housing the animal - not supporting it financially.
And to twelvetigers, I agree about the pet ownership thing and I actually wish I had owned a dog at some point. I think there is a lot of body language that you learn when you own an animal, and I feel "behind" most people in reading a dog or cat. Problem for is is that I've always had rabbits, so I don't want to own a predator until I have a big house to keep everyone separate if necessary. I think if someone has really never owned an animal, even as a kid, it would be really hard to relate to the bond with an animal, the financial side, or the responsibility.
ETA: Since I'm on my foster soapbox I will add that animals coming from shelters can be very stressed and consequently are often sick. Be aware of this if you have your own animals, and consider fostering a different species than you own (like foster a cat if you have a dog) if you don't have the space to keep everyone separate. Don't be afraid to ask if there is anything unusually bad going around the shelter. There will always be URI's but sometimes some more lethal bacteria or virus makes the rounds and you don't want to bring that home to your own animals. I have a sick bunny foster right now, and it was very stressful until the vet confirmed that he was not contagious.