Agree with magda - before the site goes live is idael but not TOO optimised otherwise you could be penalised for spamming.
Hi what do you mean "after so many days"? You're questions doesn't make complete sense. On page SEO is a continual thing. It's not something you do and then forget. As you make more pages, you will always need to be doing on page SEO. Making sure links are nofollow where needed. Ensuring each page has it's own title and meta description helps. You can do this with WordPress.
It depends on fast you do well on on page optimization. It may varies depends on how big you handling website is, how well you know on keyword research, etc. There no definite time for that.
I think it must be the part of your website development. You should take care of all the onpage factors while development phase of the website. It not the good Idea to develop a website and then make alterations in the website. You should make a concrete plan of what your websites objective will be and you should design it accordingly.