You don't need to turn it at all.
But, the typical wisdom is that the more frequently you turn it the faster it will turn into the black stuff you want.
Don't waste your money on a tumbler.
Just make a big pile - then eventually you can also have dividers for different maturity piles.
(Discarded pallets work great for that)
I've grown potatoes and pumpkins on/in a pile that was never turned during the growing season.
Morning glories also sprouted...
gardening.stackexchange.com/users/23/Tim
I always have several compost piles going.
(We have two horses and a bunch of chickens, so between that and the garden
there's lots of feedstock for the piles!)
You don't need to turn the piles at all if you don't want to.
They'll take longer to compost, and the decomposition won't be as complete.
For example, the stuff on the outside won't be broken down.
A well constructed pile will heat up pretty quickly -- mine go right up to 120-130
°F when I get the recipe right.
(Lawn clippings and horse manure with bedding are a good combo.)
I'm a compost junkie, so I went and got a thermometer with an 18" or so long probe.
When I'm trying to keep a hot pile, I go out daily and poke the thermometer in.
If the pile temperature is dropping, it's time to turn it.
This can happen after a few days
(it depends on what your ingredients are).
When I turn it, I often water it (the stable bedding we use absorbs a huge amount of water).
But I don't often have the time and energy to keep the piles constantly hot.
So I turn them every couple weeks, or months (whenever time and energy strike at the same time).
Sometimes the piles get big, so I just push them up with the tractor -- not a perfect mixing, but it gets some air in there and they heat back up (enough to be steamy for a couple of cool fall mornings).
I've never used a tumbler, but it seems like you should be able to turn it 2-3 times a week to keep it well aerated.
Again, checking the temp will tell you when it needs a good mixing.
gardening.stackexchange.com/users/51/bstpierre
If you've got a lot of air going through your compost, then you don't need to turn it as much.
One good way to do this is to raise the heap off the ground by putting pine branches underneath it inside some sort of cage.
(Source: Tips for the Lazy Gardner )
If you're not going to turn it, make sure you cover your greens in dirt every so often (reminds it what it is doing).
gardening.stackexchange.com/users/10/Peter Turner
You should turn it to get air in, and to mix stuff up (rather than a lump of grass cuttings, a lump of veg peelings, etc).
We have a tumbler and don't turn it anywhere enough out of simple laziness.
combined with too many fluids this can result in a smelly anoxic black mess.
I think the tumbler was meant to be turned daily, but I usually try to do it every time I put something in.
Mounds require a lot more work to turn
so I think people usually do them once or twice a year?
gardening.stackexchange.com/users/69/winwaed
Turn your pile weekly or every 7-10 days.
A good pile has volume (3 cubic feet is a good size).
5-6 inches of brown materials by volume and 2-3 inches of green materials by volume.
Then mix, add a sprinkle of dirt or already made compost (an innoculator) and water, mixing well.
Then begin the layers again until you reach your desired height/depth.
Then water and turn. Should feel like a wrung-out sponge.
I use a pallet system and have 2 bins right now.
One for turning into each week.
I also cover with black plastic.
Use corn or sunflower stalks for aeration in the pile too.
If possible, leave your grass clippings on the lawn.
It's better for the grass.
Typically, one finds more green materials than brown (except in the fall/winter).
Paper towel/TP roles, wet cardboard (non-shiny), coffee filters, egg shells, hay, straw, lint, vacuum cleaner bag contents are good sources of brown, along with your bruised leaves (I use the lawnmover to chop them up).
I built a pile on Saturday afternoon and it was already hot Sunday morning.
If you turn regularly and have the right amount of air, water and the contents, you can have compost in 6-12 weeks.
Usually, you let it cure for another 1-2 months.
This means chopping up things small and not just tossing them in, food scraps included.
gardening.stackexchange.com/users/1144/DD - Master Composter
If you have a compost heap, how often should you turn it? Does it make a difference if it's a simple heap (a mound on the ground) or in a tumbler?  Alternately, is it even necessary to turn the heap?
How often do you need to turn the compost heap?
gardening.stackexchange.com
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gardening.stackexchange.com/questions/93
gardening.stackexchange.com/users/79/Michael Todd
You don't need to turn the pile, especially if it has a lot of air in it, but it will help it to compost faster if you turn it regularly. You can turn it once every 7-10 days.  You don't need to turn the piles at all if you don't want to, but if you turn regularly and have the right amount of air, water and the contents, you can have compost in 6-12 weeks.  If the pile temperature is dropping, it's time to turn it. You should turn it to get air in, and to mix stuff up. If you've got a lot of air going through your compost, then you don't need to turn it as much.  Suggestions of how often to turn the heap range from every week to once or twice a year.
 You don't need to turn the piles at all if you don't want to, but if you turn regularly and have the right amount of air, water and the contents, you can have compost in 6-12 weeks.   If the pile temperature is dropping, it's time to turn it. You should turn it to get air in, and to mix stuff up. If you've got a lot of air going through your compost, then you don't need to turn it as much.   Suggestions of how often to turn the heap range from every week to once or twice a year.