As the others said, it can vary but I have found that one of the fastest ways to grow an audience is to give away some related knowledge. For me, this was creating online courses to provide for free in the areas of photography and online marketing. It does not take a lot of time to do a screencast or a couple of videos teaching something and there are plenty of marketplaces for online learning that have students wanting your content. From two courses I have just shy of 30,000 students and it's still growing. Once you have a decent audience who you trusts you because of what you have taught them, you can ask them to check out your offerings at other marketplaces.