Hi Vijay Raghunathan
The following information is from Turnitin's help guides:

Some assignments may allow students the ability to overwrite their previous submissions until an assignment's due date. This option is activated by an instructor on an assignment-by-assignment basis. If resubmissions are not enabled, your instructor must manually delete your previous submission to allow you user to submit your second file.

If resubmissions have been enabled or an instructor has deleted your first submission, resubmitting a paper is handled in an identical manner as a first-time submission.

You are allowed three resubmission attempts where the Similarity Report will generate immediately. After three attempts, you'll have to wait 24 hours before a new Similarity Report can be generated.