Google crawler help your site by crawling on search engine. But how often Google crawler crawl your site it is not specified by Google. But you can set Google crawl rate from Google webmaster tools. In addition if you want Google to crawl immediately then you can use Fetch as Google which will work instantly.
Google is very rich by their strong algorithm which will determine how much to crawl every site. This algorithm try with their best effort to crawl our site all pages without crushing server's bandwidth.
Remember that if Google crawl a site most often then our site server become slower. In this case changing Google crawl rate control work best that will help to increase your server bandwidth. Actually the crawl rate means the total time that use by Googlebot to crawl by the site.
However there cause some problem in case of sub-domain. Suppose my blog address is www.bloggerspice.com and if I create a sub domain like http://blog.bloggerspice,com then Googlebot will cause some problem If you change the crawl rate. Because by default Google crawl very fast but in case of subdomain with custom crawl rate it is bit slower to crawl by Google.
There are other restriction about the crawl rate for a sites, if you are trying you crawl a nonroot level domain (e.g. www.bloggerspice.com/folder).
Anyway now we will learn how to change the crawl rate on Google webmaster tools. Just follow the below steps-
Step 1 Go to Google Webmaster Tools and Sign in to account
Step 2 Now click on any site for controlling crawl rate (If you have more than 1 blog site).
Step 3 Click the Gear icon and then click on Site Settings.
Step 4 Under Site Settings select Limit Google's maximum crawl rate.
Step 5 Now more option will visible below with a scale changer option. Hold it and move right or left for changing the crawl rate.
Step 6 Finally hit the save button.