The answer is: it depends.
If you're like most companies, the goal of doing inbound marketing is to increase brand awareness and get more leads. You want interested prospects to come knocking at your door (which, by the way, shortens the sales cycle significantly).
In order for prospects to know who you are and feel confident that you can help them, you need to put your name out there in some way.
Traditionally, companies have used strategies like direct mail, cold calling, trade shows and the like to spread the word. The issue with "outbound" marketing is that you're pushing yourself on people. You have limited information about the prospects and where they may be in the buying cycle - or if they're in the buying cycle at all. And usually, the type of messaging produced in traditional campaigns borders on fluff (accolades and how great you are). Prospects have become very good at blocking those messages.
Inbound marketing focuses on helping companies establish themselves as thought leaders in their industry by providing valuable content like blogs, ebooks, and infographics. It focuses on educating prospects and guiding them through the buyer journey. It's helpful and not self promotional (too much!).
The cool thing is that as long as your content is relevant and valuable to your target audience, prospects begin to get to know you. They start to trust you. So, when they're ready to move into the latter stages of the buyer journey, you're the first place they go. Bingo bango! Qualified lead.
Once you decide to shift your marketing resources from the traditional type to inbound, the length of time it takes for you to start seeing the leads come in depends on how much - and the quality - of content you have. Here are a few scenarios:
Traditionally old school and have no content
If you're website is marginal and you don't have much in the way of articles, ebooks, case studies, etc., it may take as long as 6-8 months before you start seeing significant results. Creating good content takes time and a shift in your business culture - but it's totally worth it! The content you create can continue generating leads for years, so it's a very good investment.
It also takes time to start building up credibility as a thought leader. For example, if you're starting with a brand new blog, it will take some time to build a following. There's truth to the old saying "if you build it, they will come!" The key is to be patient, consistent and don't give up.
More progressive and have some content
In this scenario, you have a fairly decent website. You've been creating some content like case studies and presentations, but you haven't leveraged them for lead generation online. Companies in this situation can expect to start seeing results in 3 to 6 months. Once you lay the groundwork, your current content gives you a starting point to build upon. It will still take time to build a following, but you'll get off to a faster start.
Traditional vs. progressive corporate culture
Your corporate culture also has a huge impact on how quickly you can start seeing results. If the concept of content creation is embraced company wide, you'll be surprised at how fast things start to happen for you.
Conversely, if your people don't understand the value of creating content, don't want to be a part of it and want to keep doing the same-o same-o, it will take longer to start seeing the shift. Don't be discourage though if that's the case - once they start seeing how inbound marketing works they'll start to believe in it!
The reality is that inbound marketing results don't happen overnight. However, this type of marketing can get you greater return on your investment over time. The longer you wait the longer it's going to take, so don't put it off any longer! It's up to you to make it happen.