This paper presents a study on the impact of the length of formal exposure on the grammatical learning among foreign language learners. Past researchers claim that age plays an important role in language learning; however, in more recent studies on language learning, besides age, another factor that has appeared to be significant is input, particularly in the instructional setting. Previous studies have shown that access to second or foreign language input affectsthe language learning process. This study mainly focuses on the influence of the length of formal instruction on the grammatical performance of EFL learners in Iran. Two groups of Iranian learners with different duration of formal exposure were tested using a grammaticality judgment test. The first group was exposed to formal English for 792 hourswhile the second one had the opportunity of attending English classes for 1272 hours. The findings show that the group with more exposure time obtained higher scores which suggests how useful extending the formal teaching/learning time can be for foreign language learners speciallyin the Iranian EFL learner context.