Hello there! You will get 9 months of use.
Divide 90 by 15. This will see how many months of service you can get by dividing how much you paid by the monthly fee.
90/15 = 6, so you get 6 paid months of service, but remember that you also have 3 free months, so in total you get 9 months, which is your answer.
I hope this helps answer your question, have a great day! :)