Web marketing tends to be very time consuming, especially when you consider how vast it is. Pulling together all the components of SEO, content, website architecture, analyitcs and social media, you can be looking at 5-20 hours per week. The larger the site the higher that number goes. Social media itself can be particularly time consuming to do right, since that involves not just content generation but spending time engaging online with your potential audience and online thought leaders. I would say, however, that on a skill level, that's the easiest to achieve, if one has the knowledge that is required. But every area of web marketing requires a good deal of knowledge and a great deal of skill to ensure success.