I've only had a site blacklisted once. My father ran a WordPress blog on shared hosting and got hacked (probably weak password or vulnerability in one of the plugins or WP itself, who knows). His site was pretty quickly blacklisted, and even after he scrubbed it, leaving just a basic index.html ("we are coming back" type thing), it stayed blacklisted for at least several days. I am sure others have more experience with this, I've just been lucky.