Almost all of these are limited time offers. I know the Random House freebies (16 mostly romance genre books) are at least all month (some have said possibly all year, but that may only be on their own web site). These three just popped - of these, Samhain titles tend to be free for a week or a month. Still Waters was rumored to be Sunday only, but still shows free today. And the Lee Child book just showed up, so no idea how long it will be free.
Keep in mind that they have announced free books on the Amazon Kindle Blog, links and all, saying they would be free for several days, then had the blog entries yanked and the books either go up in price or get pulled completely, with no explanation. In general, if there is even a remote chance you (or anyone on your Amazon account that shares or will share a Kindle with you) will want to read a free (or bargain) book, you should buy it as soon as you see it.
The shortest time sales I've seen have been less than an hour....
Karen
http://booksontheknob.blogspot.com/