In order to read fluently, children have to form orthographic representations. Despite numerous investigations, there is no clear answer to the question of the number of times they need to read a word to form an orthographic representation. We used length effect on reading times as a measure, because there are large differences between long and short words for unknown words, differences that disappear when reading frequent words. Two lists of new words (half short and half long) were presented to 102 second-grade to sixth-grade Spanish children, to be read aloud six times. One of the lists was presented as part of a story and the other in isolation. Comparison of the length effect at the first and sixth exposure showed large differences between short and long words in the first presentation and small differences at the sixth presentation, indicating that Spanish children develop representations of words after just a few repetitions.