Google's crawling rate tends to be linked to how important they think your website is. Sites like Digg and the New York Times are crawled every few minutes. Weak websites will be crawled every few days.
Your rankings will only be impacted for the pages that were blocked when Google tried to visit it. If you caught the problem quickly then very few pages will have a problem. Once you remove the block the rankings will return to normal after the next crawl cycle which is dependent on how important your website is to Google.