An analysis done by Nation (2014) leads to the conclusion that readers in English as a foreign language can gain about one-half a point on the TOEIC test for every hour of independent English reading. A statistical analysis of progress made by seven adult acquirers of English living in Japan was performed to confirm this conclusion: All were intermediates, but there was considerable variation, with TOEIC scores ranging from 220 to 705. All engaged in self-selected reading, and took pre and post TOEIC tests. Hours spent reading was shown to be an excellent predictor of gains on the TOEIC, and the rate of improvement was shown to be nearly exactly the same as that reported by Nation.
Krashen, S., & Mason, B. (2015). Can Second Language Acquirers Reach High Levels of Proficiency Through Self Selected Reading? An Attempt to Confirm Nation's (2014) Results. International Journal of Foreign Language Teaching 10 (2): 10-19.
Available at: ijflt.com OR http://sdkrashen.com/articles.php?cat=2