We are seeking a guardinship of our niece and both parents are in agreement. The school stated she can't enroll without this. How long is this proce

You need not wait for guardianship to enroll your niece in school.  Her parents can sign a special Temporary Durable Power of Attorney, revocable at will, allowing you to enroll her in school, take her to the doctor, etc.  A local family law attorney should have this form.