How soon you can check the chems after an addition depends on how well your pump circulates the water. I usually wait a couple of hours before testing again.

As to how often to test, I check pH and FC/CC every day, TA once a week, CYA once a month after I have it where I want it to be. Because our pool has a lot of splashout from the dogs we refill almost daily and have considerable dilution of CYA and have to adjust it more frequently.