7/15-9/10

Assuming you mean July 15 to September 10 of the same year:

• Inclusive days (counting both endpoints): 58 days
• Exclusive days (not counting July 15 as a study day): 57 days
• Roughly 8 weeks and 2 days (inclusive) or 8 weeks and 1 day (exclusive)

Would you like this turned into a study plan for a specific subject or exam? If you tell me the year, the subject, and how many study days per week you want, I can draft a day-by-day or week-by-week schedule.