Link building is an important part of SEO. I would say you can't expect high search engine rankings, till then you don't have some quality back links for your website. Backlinks requirement depends on the competition on keywords.
Link building (of course quality links) is an essential part of SEO and Its help to ranked your site in a good position in the search engine. Link building helps to get traffic for your site and pushes your site towards higher PR.
Yes 60 - 70% of SEO is link building and anyone who says it is not the most important thing you will do, doesn't know anything about SEO.
Off page SEO is all about link building, quality backlinks that does matter in SEO. Without link building getting good position for competitive keyword is very hard.
Link building is just a part of the SEO experience. Getting your pages indexed and ranking for certain keywords requires a lot more than 100s of backlinks.. but certainly it's a large step forward!