I have made some changes on my main page (new keywords and content changes reflecting new keywords) so I am now wondering how long it will take for Google to take notice of my changes (SERPs). My main index page get crawled about 4 times a month.
Hard to say, depends on your pagerank and backlinks I think. My largest sites are constantly being indexed, but my smaller sites see Googlebot from once a month to several times a week.