I personally take all the shavings/straw out every 4 months or so and hose down the coop (My neighbors may complain about the smell, they live really close to my backyard), but you can completely take it out once a year, but scoop it once a week and top with fresh shavings/straw.
I clean my coop out completely about twice a year. But, my birds have a lot of room and it doesn't get dirty very quickly.
Oh yeah agree with Wyandottes7, your coop looks a little small so you may want to clean it more often like 3-4 times a year.
When it's dirty? Maybe I'm just accustomed to cleaning horse stalls daily so I just tack the coop on to that. It's just another stall. Every day I scrape all the flat surfaces that have poop on them, like around the feeder platform, and cover the poo under the roosts with clean sawdust. I put enough in each week that I can just sweep new on top where needed. I clean out the piles under the roosts at least weekly. Whenever it's needed.
I have 11 chickens with a 12'x24' run and 8'x4' coop. I keep shavings in the coop and nothing but the ground outside (mostly dirt with bark, rocks, etc). I do a "mini-clean" of the coop every day that mainly includes keeping the nesting boxes clean. I remove any poopy/wet shavings and replace with fresh shavings. Once every other week, I do a slightly more labored cleaning - my girls tend to "mess up" 2 parts of the coop more than anywhere else, so I shovel out these parts completely, add some fresh shavings, and mix it all together. 4-5 times per year, I clean out all of the shavings and replace with complete new shavings.
I have two coops: a small one that only holds four hens, and a larger one. I clean the small coop out once a week, since there is less bedding in it and the droppings build up faster. However, I only clean the large coop out completely once or twice a year.