Once a week for us, sometimes more often if they stink or roll in bugs or bird poop or whatever. Sammy also pukes sometimes when her tummy is empty and the only way to remove that smell from her beard is to dunk her. Gotta love 4am baths!
We use Buddy Wash - LOVE IT! Buddy Splash and Growl & Meow for in between. We do faces & eye goobers every day, teeth every other day or at least on bath day and paws/butts as needed. Usually by the time the paws are crusty and the butts are stinky it's bath day anyway.
Mine go to the groomer every 4-6 weeks but they are cut short and grow out quickly. I can and have done it myself but I love my groomer so we just go there.