This question is like "how long is a piece of string". It could be anywhere from 2 weeks to a few months. This is dependent on your competition and how much work is being carried out, also how often search engines update their index. What takes time is for links to get indexed. When I submit your website to directories these sites can take as long as 4 months to accept links. What is important is consitancy. There is little point have a couple of months work done then stopping. SEO should be part of your ongoing marketing budget. A couple of hours a week of SEO could be all your need. After 6 months to a year you will really notice the traffic and benefits of SEO.