Also, I am a big proponent of the towel wrapping method for ensuring compliance.
When we first got our cats, the vet tech told us about that technique and called it "kitty burrito".
We adopted our cats as an adult pair (about 3 years old) from the person who originally adopted them from the Humane Society. For whatever reason, he didn't trim their claws (or maybe did it only when they were sleeping or something), so they weren't used to it. The female wasn't happy but was docile enough to put up with it. The male had to be wrapped in a towel and one paw extracted at a time. Fortunately, it wasn't that long before they both figured out that "claw clipping = treats". Almost nine years later, claw clipping is a non-event, except that sometimes the male still swishes his tail and complains.
As for timing, about once a month, or whenever we notice the claws are getting sharp. We have a few scratching posts and both cats use them regularly; I don't know if that makes any difference in the timing or not.