It's very important to keep your pet rat's cage clean and hygienic. Rats, generally, in most cases, will learn to eliminate in one particular area of the cage, and often times this allows the ability to keep their home environment a bit cleaner. Because they do spend the majority of their time in the cage when they are not directly supervised, or when somebody's not around to handle them, or play with them, or have them out of the cage, it's important to make sure that that environment stays very clean, both for the pet's health as well as to make sure that your home doesn't have specific odors, or doesn't appear to be dirty because of a filthy rat's cage, which we never want to happen.
Depending on the number of rats you have in the cage, and depending on whether these are rats that can be litter trained, or that can eliminate in certain areas of the cage, allowing you to keep it cleaner, it may mean weekly or twice weekly cage cleaning. Like I said, you'll often have to gauge it depending on how many rats you keep in there, how quickly the bedding become soiled, how often that they make a mess, dump their food, or any time that there is any type of mess in the cage is going to dictate how often you need to clean it. There's no right or wrong answer on frequency, it depends on what keeps it clean, what keeps it odor-free, what prevents there from developing too much ammonia from too much urine, and also allows the rat to have clean bedding to dig in and burrow in, providing that they're not having to only be in areas where their cage may become soiled. It's important to use non-toxic or safe cleaners.
If you're going to use bleach, it should be diluted to a ten to one dilution, and the cage should be washed with that. Sometimes people find that very hot water with vinegar is cleansing enough. Some people find that just changing enough bedding, which absorbs all the urine and which will keep all the feces contained is enough as well, and they'll be able to dump and change the bedding in the cage, wipe it down, clean it, and reassemble the cage with fresh, clean bedding. So overall, cleanliness of the cage is going to be contingent on the number of rats you have, their types of behaviors, how well you can keep the cage clean is going to be contingent on your commitment to making sure that your rats have a clean and sanitary environment to live in.