As with any other type of SEO, there's no simple answer to this question. In most cases changes to search traffic can be seen within 4-6 months of context-optimization.
Contextual relevance and readability are two important factors, but there are many additional variables which influence search performance. These variables include the specific keyword being optimized for, the competitiveness of that keyword, and a number of traditional ranking factors.
In some cases, Wordmetrics will show results in as little as a month. As a general guideline, it is recommended to wait 4-6 months to see results.