PageRank is updated by Google 4 times per year. So, technically the answer to your question is; the time until the next PR update (2 months).
Actually, It depends on you. How fast, do you want to get good page rank in the Google. Google likes quality content. So if your content is good enough, then it will help you to get page rank. Follow SEO techniques to get good page rank. There have many techniques of SEO.. i.e. white hat seo, black hat seo, blue hat seo. If you follow black hat seo, then you will get page rank faster then another way. But it doesn't give better result in the future.
Continue to build quality backlinks and create fresh content for your site and it shouldn't take long. Good luck.