Google regular visit websites or blogs and see what is happening new there and indexing that data in it's database. Whatever we are updating in our website or blog Google crawl and index all that and keep them on different parameter which it has made for them.
How many times Google is coming to your website or blog for crawling depends upon quality of your website or blog and quality content.
Everyone wants to go there where is quality even in real life we are doing the same thing. If i say Google would be visiting seomoz regularly even hourly and other side a new created website or blog may be visiting by Google once in a month. Because Google loves only quality content with new updation.
At the end i want to say that if you want Google crawl your website or blog so many time google will do this but the condition is that you would have to served a quality website or blog as per the given parameter by Google.
Now how to check whaen last time Google crawled your website or blog ?
It's very simple just put the website name in Google search bar and click on cached link there.......
Date displayed when last time Google crawled your website.