Yes, full time SEO is required until Penguin update rolled out by Google. Post Penguin update ,Google knows what is Natural Link Building and What is an artificial link building. If people like your website and are talking about you over the internet Google Penguin would love to show your website among the top results.
On-page optimization requires professionals to investigate your website and make it Search Engine Friendly, most important factors here are: Title & Meta Tags, H1, H2, H3, Loading Speed, w3c validated html and css & Alt tag on images.
Gaining site traffic is not a cakewalk, as it involves study of various technical aspects that WWW keeps putting across .You actually need a Full Time SEO or Part Time SEO – it depends on your business objectives and target audience.