There is something called Google Sandboxing. Even if your website is doing good, it might not appear in the search result in first 6 months heavily. Google wants you to build an online reputation before trusting the webpages by displaying it in search results.
As said above, there isn't any defined period to get into google rankings. It clearly depends on what industry do you belong to, how strong is your competition and how aggressively or what quality of backlinks you create.
Now a days, it seems things does not work for long time, for new webmasters seo has become a headache. As nothing is working. It is so disappointing, we cant tell anything how much time it will take to rank on first page.