The vet hospital that I take my cats to is open 24/7 because it's also an ER hospital. Even though they do charge more than many in my area, they're always busy. They've even outgrown their current hospital so they'll be moving to a new larger building in just a few weeks.

I always take my cats in for annual check ups and if I notice that they're not feeling well or not acting normally then I make sure to take them in for a vet visit. Depending on what's going on, I'll only wait a few days before deciding if it's going to get better on it's own or not. Sometimes I think I go too often but I'd rather get them checked out than worry if or when they'll get better. If I don't have the funds then I just use my credit card because their health is very important to me.