How often varies with the pig, On average once a month.
I use a good sharp pair of Toenail Clippers you can buy at a drugstore, not the small ones but the good size ones.
As for how short?
It's easy if yours are like my two girls and have Clear nails, You'll be able to see the quick in the center or the nail Clip just below it, If you should happen to hit it make sure you have some corn starch handy to help stop the bleeding.
Or you can obtain some Quick stop for birds at your local pet shop, this is what I use and it works almost instantly.
The type I use also has a topical pain killer in it that is Bird as well as Pig Safe.
If your pigs have Black nails it's a little tricky but easy enough.
from the center of the length of the nail on down to the tip, cut at about if not a little less then that distance from the tip and it should be fine.
It can be a little tricky if you haven't done it before but once you get the hang of it, it's not so bad.