I had changed the content, meta tags, robots including Google verification code(new code) for my site which is already indexed by Google and found in Google main crawling section, but after making complete change except web design of my site, whether Google will consider my website as a fresh crawl ? what is the duration taken by Google from fresh crawl to main crawl ?
there is nothing like fresh and main crawl. Whenever you change your website's page and upload it to server wait for at-least 2 or 15 days. When google-spider will visit your website and find updated content/page that will crawl automatically.
their is nothing fresh crawl or old crawl whenever you change content or webpages Google again crawled your sites
Google spiders crawl the web regularly to rebuild the site index. The crawling process in Google is algorithmic which will determine which sites has to be crawled and how many pages the spider has to fetch from each site. This process is based on many factors like page rank, site links, URL parameters, and all these factors can affect the crawling period of an individual site.