Have to ask why you are assuming it won't activate for 90 days? Are you starting school July 1st? I ask this because scholarship money usually comes in @ Oct 1st. Granted maybe they said it can take up to 90 days, but for DS it usually was within 6 weeks of school starting.
Make sure you stay on the ball regarding the pay, because occassionally there are paperwork issues that delay the reimbursement. Many times it comes down to numerical error during inputting...i.e. number imposed for SS, DOB or checking account. Sometimes it can also be a DodMERB issue where you are not cleared.
First lesson in the military, never assume anything. Always be informed. If everyone else is getting their stipend, and you are not, don't wait forever to work the situation, give it a few days, maybe a week or two, but after that start talking to your det. It is much harder to get months of back pay than it is to get a month of back pay.
The one that has always taken the longest to get reimbursed was books, that usually took about 8-10 weeks.