You can harvest vermicompost (VC) anytime you need it. Harvesting techniques differ depending on that need and also depending on personal preference. Some people like to have high percentage castings (just the worm poo) with little other organic material. I personally use "chunky" VC with maybe only 50% castings. The former requires using screens down to 1/16" if you want that fine coffee grounds look. The latter sometimes doesn't require any screening.

Let's say you need a handful of VC for one of your potted houseplants. You would simply make a pile of some of the more processed material in your bin and put it under sunlight or a lamp. Any worms will burrow to the bottom of the pile fairly quickly. Remove the material that doesn't have worms and you have worm-free VC.

This VC may have babies and cocoons, so you may want to spread it out and remove those. Or you could use a sieve with 1/16" screen (window screen) and remove practically all babies and cocoons. The resulting VC would be potent. Use sparingly.

The advantage to this piecemeal harvesting is that you don't disturb the entire wormery. But if you'd rather harvest the entire bin all at once, the above method could easily be scaled up. There are, of course, other methods. This is just one example.

Andrew

Another idea if you have a lot of time is to use a system where the worms separate themselves. You can push all your current VC to one side of the bin, add some sort of barrier that has some holes in it and start on the new side (more on starting in a minute) or you can put a tray that has holes drilled into the bottom of it to the top of your current bin.

Take any large chunks of obviously unfinished food and bedding out of your current bin and put them in the new bin/side. If the worms are congregating in one particular area, you could speed things up a bit moving handfuls of worms over to the new spot. Then only feed the new part. One of the reasons to take anything still obviously food out of the mature side is the fruit flies you mentioned. If there isn't any fresh food on top, they shouldn't like the old side that much and you may not need a thick layer of bedding on top.

The last way you can do this kind of harvesting is start a new bin with food, bedding and the same larger chunks of stuff from your current bin. Take something like a large trash bag and put holes in it with a pen, drape it over the new bin and put the old (mostly finished) contents of your old bin on top of all of that. The idea behind all of these is that the worms will migrate to where the food is. There will almost always be some food left in the older side and not all of the worms will move, but then the stack and sort method that Andrew mentioned is easier. Worms eat bacteria that eat the food and bedding. The bacteria also eat worm poop, so even in well finished VC, there's usually some food. The obvious down side to these methods is that they take a lot of time.

In the pictures, the huge bins of well processed VC have probably come from a flow through bin or a stacked system. Either that or from someone with a huge bin and a sorting process (screen or something.) Also, people with more worms than they know what to do with (gee, I wish I had that problem!), sometimes just stop feeding one of their bins for a few months and let the worms work through what's there. If you wait long enough, you get pure worm VC, but all the worms die. Most who use that system stop short of that and remove some of the worms. VC doesn't have to be that well processed to work!

Once you've been worming for a while and are thinking of adding a new bin, you can check out flow through bins if you're interested. One source is a page of links to all sorts of DIY flow through bins at http://vermicomposters.ning.com/forum/topics/diy-flow-through-bins-a

I live in a studio apartment and have the small system that's 4th from the top. This site and others usually have a search function and entering 'harvest' will get you LOTS of ideas. Bentley at Redwormcomposting.com has a video of his 'turbo light' harvesting method at http://www.redwormcomposting.com/worm-composting/turbo-light-harvesting-method/ which is a faster version of what Andrew put above.

HOWEVER, if you're only a few months in starting with 20 worms, you'll probably want to wait a long time before doing serious harvesting. The theoretical doubling time for worms is about 4 months. I started with 2 ounces of blue worms (more than 20, but not by a lot), and didn't harvest for at least 6 months. If you need some now, do what Andrew suggested and leave the rest of your bin be. The less you disturb them, the better off they will be.

I do not know why for sale systems say worms crawl up. Worms do crawl down. They will even crawl down and out of a good system. Less worms crawl up.