Depends a bit on the concept of site and the purpose, i.e. 'shopping' vs. 'content' site. For the shopping sites I set a budget for marketing that initially way surpasses any revenue - which is naturally very low for a new site. I usually take a 9-12 month time frame during which I'll try and break even. I find it important to give a new site a substantial initial boost, after which better returns come in in a shorter time.

For content sites I spend considerably less and focus more on letting the site develop by itself. I do buy links to get G to visit, but generally use non-$$ ways of marketing.

In the long run I spend about 25-30% revenue in on- and offline marketing for a shopping (=non-content) site.

Hope this helps