You sign in once, and it stays logged I think for close to 5 minutes then resets for your protection. I'm guessing on the time because I can one click buy a few apps consecutively, but if I show no activity for a few minutes it prompts me to sign again. I don't mind really, it saves me from buying things accidently.
It also protects your wallet, say if someone barrow your iPhone/iPod Touch and they want to get a free app, but then they need a password for it, you type that in. After that, they accedently purchase something else that costs $100 and your iDevice doesn't ask you for the password again.
Probably caused by the pool of complains about kids spending too much on in app purchases! There's an article about it not long ago!
