I've been sending new copies to my brother and the others every few days. That might be a bit extreme, but depending on what I'm working on at the time anywhere from, I dunno, 1 to 2 weeks, especially if I've fixed some naughty naughty bugs, because I hate having them up on the web. It's an OCD thing or something. It makes my skin itch. Unless they are LCS and Armok bugs, apparently those can be on the web for a long time. If rapid releases end up causing too much confusion with save file versions and so on it might be stretched.
That's something to consider anyway... there are world files which can accommodate many successive fortresses. Ideally, these files could last a long time. But it's still a bit early in the development process, so both bugs and the rapid addition of new features are working against world longevity. People probably shouldn't become too attached to particular worlds at the moment. It's also good to back up frequently, since a bug can corrupt one of the many files and then slowly cause the degeneration of your universe. Now, you'll be able to load old worlds with the new versions (with rare exceptions), but since world generation often incorporates new features your old world will be missing some things, especially new creatures and objects and so on. It'll be up to the individual how often they want to re-gen.
I imagine the process will work itself out a bit once we've tried it.