The robots.txt from news.ycombinator.com reads as follows:
User-Agent: *
Disallow: /x?
Disallow: /vote?
Disallow: /reply?
Disallow: /submitted?
Disallow: /submitlink?
Disallow: /threads?
Crawl-delay: 30
So nominally you should feel free to set up a scraper that crawls one non-disallowed resource every 30 seconds.