The by-laws, as with all information regarding the organization, need to be available at all times.
I'd give new members a copy upon joining and be sure to keep a couple copies on hand, in the event you're ever asked to produce it.
We have a PTO binder in our school office with our by-laws, code of conduct, monthly minutes, treasurer's reports and any printed materials we send out to parents and staff. It is required that we, as an organization, have open books and this helps us make it more widely available to anyone who should be seeking it.
In terms of changes, changes need to be made when the current by-laws no longer work. With a bit of great wording and explanation, changes will be few and far between.
I'm not sure I understand exactly what you mean by "available."
We have ours posted (by a link) on our website and each board member has a copy. If anyone else wants a copy I gladly share it as well. As an incorporated non-profit our by-laws are basically public record anyhow.
As to how often to update them. They should only need updated when they no longer work. However, they should be written well enough to start with that changes aren't necessary but once in a blue moon. I know with ours to make a change is not a simple process so we made a point of writing them as good as possible the first time, and basically those on that committee put a mental time frame on them of having them work for at least the next 5 to 10 years before major changes should be needed. There are a few small items that have hung us up or have ended up not being as effective as initially thought due to changes with the school, membership, life, etc. We have yet to run into a large enough problem that we feel a rewrite is necessary for anything though. I expect it will happen at some point though....
