I say, and with God's success: If he was not aware of the impurity and was using the water: then it should be considered: If the object that fell into the water has swollen or decomposed: it is ruled that the well has been contaminated for three days and nights; because swelling is a sign of the passage of time, thus he is required to repeat the prayers of that period if he performed ablution from it, or bathed from impurity, or washed his clothes from impurity with it. However, if he washed his clothes from impurity with it and did not perform ablution from it, he is only required to wash them properly; because it is akin to the presence of impurity in the garment, and he does not know the time of its occurrence, nor does he repeat his prayer. However, if it did not swell or decompose: it is ruled that the well has been contaminated for one day and night; as a precaution. Refer to: Rad al-Muhtar 1: 147, Al-Wiqayah p. 102, and Maraqi al-Falah p. 42, and God knows best.