Assuming you want a thorough proofread:
At a rate of 250 words per hour, it would take you 60 hours to proofread 15,000 words. This calculation does not take into account any time that may need to be spent researching particular language usage, grammar rules, or style issues.
If you proofread at a rate of 500 words per hour, it would take you 30 hours to proofread 15,000 words.
It is important to note that the time it takes to proofread a document also depends on the level of detail required. A light proofread may take half the time of a thorough proofread.