as I am keeping doing SEO on each page, I went back to add additional information on the page every now and then, however, I find the page is still indexed as the original page. Does google automatically re-index all the page? if it is, how often google does it? Or do I have to do something to make google reindex? Thanks for your advice.
It could take a day or a month, it just really depends on Google's algorithm and its really hard to understand..
Of course, when you save a page, the new page will show to the users. If you mean how long it takes for new content to influence on SERPS, it could take a couple of weeks or more.