General guidelines are…it depends
Active runners are fit, so they mostly just need maintenance. But you also asked about getting a horse fit, which is different than maintaining an already fit horse.
For maintaining fitness, some horses need or like more work—daily galloping, say, with a faster work every week or 10 days—if there’s more time between races. If there’s a short time between races, they likely won’t have a faster work at all. Some do better instead with lots of jogging or swimming and can stay fit that way, while others need to do more to stay fit. If they are stretching out in a longer race, you might do more longer gallops. Some trainers like to do a longer slower work, while others prefer a shorter, faster work. It depends on the horse’s running style, too—a speed horse versus a come-from-begins runner, e.g.
You can tell how ready the horse is by observing them. Are they happy, tucked up, alert, feeling good? Or blowing hard, weary legged, off their feed, thin or fat? What does the rider say? The groom? Hot walker? You should also be equipped with common sense, superior observational skills, and knowledge about the physiology of the horse.
Meanwhile, conditioning plans, such as they are, will also change depending on whether the horse has a physical issue and if so what kind and whether and how it can be managed. How long and how tough the next race is, and when it is. Some races come up regularly, some not at all, and you don’t know when the next one might go, so you end up having to train more until you get your race or pivot. How many races in a row how quickly will dictate your schedule. What the owner’s goals are will dictate your schedule. There will be weather. There will be people’s and racetrack’s scheduling conflicts to work around. There will be respiratory infections. There will be pressure from the front office or owners to run, and lots of surprises, and some mistakes.
If you’re really interested, I would suggest reading as much as you can in racing publications, and pay attention to what trainers say in interviews when they talk about their horses. Watch races, watch horses train. It’s something you can’t really learn on paper, I don’t think.