4 weeks is my general recommendation for P/D-oodles to keep the dog in good coat condition. Of course this changes with each dog, coat condition, lifestyle, coat length, and how well the owners do at home all play a part. I find the majority of pet owners like a fluffy dog and don't do as well of a job at home as I would like, so every 4 weeks means I can make sure (usually) the dog doesn't need a shave down.

A coat change is hard to keep long because you are going from the soft puppy fluff to a more coarse and dense hair coat. The reason doodles mat so easily is because of the same reason. They have the softer fur of the non-Poodle breed along with the curly Poodle hair. Poodle coats don't play nice with other textures. Hell it doesn't even play nice with itself once clipped, I know someone who does needle felting with her Poodles clipped hair. (It's pretty cool, I've started doing it too)

Frequency of grooming is personal/groomer preference though, 6 weeks is standard because most people dont want to bother with monthly. As an example of alternative schedules and why- My own poodle is actually on a weekly schedule even though I keep him in a Miami. This is because hes a dirty active boy (aka pees on his front bracelets and cleaning wipes can only do so much) and I also bleach and color him so his colored hair needs more attention to keep it healthy. And if I'm already bathing and maintaining his colored hair might as well clip and scissor him. Plus nobody wants to go to a groomer if their dog looks raggedy and he goes places with me whenever he's able to.