Usually the answer to "is it legal?" to wholesale and methodically copying another website (or any other published work, be it a book, movie, music, article, internet resource, etc.) is clear: of course not.
It doesn't matter if it is a one time scrape or a continuous crawl and copy; it's still copying and there are copyright laws around the world, albeit different in many countries. But still, copyright exists, everywhere.
Read more: I have a question about copyright. What should I read before I ask it? and https://law.stackexchange.com/questions/tagged/copyright
Copying something in its entirety is not Fair Use, either; Fair Use is the use of a small amount of material, not a complete work. See http://fairuse.stanford.edu/overview/fair-use/what-is-fair-use/ and https://law.stackexchange.com/search?q=fair+use
To be sure, read the TOS for the site you want to scrape. If there is no TOS, or the TOS says you can scrape and copy, go for it. If the TOS says scraping is not allowed, you may still be able to license the content; ask them.
If the TOS is like IMDB's, scraping and copying is illegal, but licensing is possible. Read IMDB's TOS http://www.imdb.com/conditions :
(my emphasis)
Copyright
All content included on this site in or made available through any IMDb Service, such as text, graphics, logos, button icons, images, audio clips, video clips, digital downloads, data compilations, and software, is the property of IMDb or its content suppliers and protected by United States and international copyright laws. The compilation of all content included in or made available through any IMDb Service is the exclusive property of IMDb and protected by U.S. and international copyright laws. All software used in any IMDb Service is the property of IMDb or its software suppliers and protected by United States and international copyright laws.
License and Site Access
Subject to your compliance with these Conditions of Use and your payment of any applicable fees, IMDb or its content providers grants you a limited, non-exclusive, non-transferable, non-sublicenseable license to access and make personal and non-commercial use of the IMDb Services, including digital content available through the IMDb Services, and not to download (other than page caching) or modify this site, or any portion of it, except with express written consent of IMDb. Additional license terms may be found in the Terms. The IMDb Services or any portion of such services may not be reproduced, duplicated, copied, sold, resold, visited, or otherwise exploited for any commercial purpose without express written consent of IMDb. This license does not include any resale or commercial use of any IMDb Service or its contents or any derivative use of this site or its contents. All licenses are non-exclusive and all rights not expressly granted to you in these Conditions of Use or any applicable Terms are reserved and retained by IMDb or its licensors, suppliers, publishers, rightsholders, or other content providers. You will use all IMDb Services in compliance with all applicable laws.
Robots and Screen Scraping: You may not use data mining, robots, screen scraping, or similar data gathering and extraction tools on this site, except with our express written consent as noted below.
Framing: You may not frame or utilize framing techniques to enclose any trademark, logo, or other proprietary information (including images, text, page layout, or form) of IMDb without express written consent.
You could license IMDB's content:
Licensing IMDb Content; Consent to Use Robots and Crawlers: If you are interested in receiving our express written permission to use IMDb content for your non-personal (including commercial) use, please visit our Content Licensing section or contact our Licensing Department. We do allow the limited use of robots and crawlers, such as those from certain search engines, with our express written consent. If you are interested in receiving our express written permission to use robots or crawlers on our site, please contact our Licensing Department.
