This is up to you, but for best results, a bin like this should be treated as a continuous system (ongoing feeding over time), not a batch system (basically “set it and forget it”). Maintenance is dead simple. As the overall level of material in the bucket drops down, add more food (ideally, water-rich kitchen scraps, but grass clippings and weeds can work too), and cover with more bedding (always maintain a decent layer of cover bedding - although, in the case of something like the weeds or clippings it’s not nearly as important).

With regular deposits of water-rich foods you may not need additional waterings, but in drier locations and when you have your buckets located close to growing plants (which can be serious water thieves) it will likely be a lot more important. What's great about these systems is that it is very difficult to "over-water" them, since they drain so well.

Wildlife pressure will vary from location to location, but these buckets (assuming you have a lid) should provide a decent amount of protection against these types of animals. That said, you should always make an effort to avoid/mask odors as best you can. Use of “living materials” (mixed in with food and/or as a cover layer), lots of bedding, and actually covering up your system (I'll come back to this in my next response) should all help. If you are in an area with significant wildlife concerns, you can also just opt for adding materials other than kitchen scraps - yard wastes and manures etc won't likely be of much interest to invaders. 

Unfortunately, there aren’t too many options (as far as I know) that would be a good alternative to a plastic bucket. Something like an old metal dryer drum would likely be a great in-ground bin, but it wouldn’t have a tightly fitting lid for added protection (not always critical with in-ground systems, just so you know). I suppose some form of slatted wooden box buried in the ground could work, but it will almost certainly will be less durable than plastic.

Circling back to covering your system - this can be a great way to not only help with masking odors (as mentioned), but it can also help to improve the longevity of the container, since it won’t be constantly exposed to UV rays and ambient conditions. A thick layer of straw, wood chips or fall leaves should work well - just make sure you mark where the system is sitting. As a side note, this is also a good way to reduce the “eye sore” factor, if you happen to be a bit particular about how your yard looks.

Unfortunately, the holes in these buckets aren’t going to be small enough to prevent smaller organisms from moving in and out. I supposed if you really wanted to keep things contained (and more critters out) you could cover the holes with a fine window screen or something like that. It’s not vital for the composting worms to get in and out (although it can be helpful).

My overall advice about this is to focus more on providing worms with ideal conditions and not worry too much about what other critters are doing. If you are seeing a lot of other organisms like ants and centipedes, it may just mean you’ve let the system get too dry.

As is the case in any outdoor system, including other in-ground systems, composting worms’ desire to stay put will always depend on how well-maintained it is. If you are providing decent conditions, with rich food materials, it will be a far more appealing environment than most other areas in your yard (certainly more appealing than the surrounding soil). But, if conditions are hazardous or the food supply becomes depleted, the worms may be more tempted to venture off in search of greener pastures.

These systems are great as in situ fertility stations when installed close to growing plants. In this case there is no real need to harvest the compost (at least not so you can put it to good use). That being said, emptying out the bucket at least once or twice a season is a great way to go. The material can either be dumped into a backyard composter (if it is full of plant roots and not all that great as a compost) or added to your garden etc.

Absolutely - but I have some important suggestions. 1) dedicate specific buckets to dog poop (and bedding etc) exclusively; 2) set up a series of these so you can rotate between them; 3) install them away from gardens, water-sources etc; 4) don't add "compostable plastic" (or any other type of plastic lol) poop bags - they won't break down effectively in a system like this. Scoop it straight in or use some form of cardboard/paper holder; 5) make sure there is a thick false bottom (of something like old wood chips, ideally) below the buck and in the bottom of the bucket itself. Just generally make sure there is a lot of carbon-rich stuff being added to these poop systems; 6) you may actually want to use bins that are somewhat larger than typical buckets (eg mid-sized plastic garbage cans), especially if you have multiple dogs or a bigger dog.