Questions & Answers about Sınavın süresi kısa.
This pattern is called the genitive–possessive construction:
• -ın on sınav marks the genitive (“of the exam”).
• -si on süre is the possessive suffix showing that the duration belongs to that genitive possessor (third person singular).
Together, sınavın süresi = “the duration of the exam.”
Turkish genitive suffixes follow four-way vowel harmony based on the noun’s last vowel:
• Last vowel a / ı → -ın
• Last vowel e / i → -in
• Last vowel o / u → -un
• Last vowel ö / ü → -ün
Since sınav’s last vowel is a, the correct genitive form is sınavın.