If a man dies without children his brother is expected to marry his widow. He's not required to; he could instead perform chalitzah and then the woman is free to marry somebody else. While his decision is pending, I assume she is not free to marry -- she has to wait for him to either marry her or decline.
Is there a deadline for his decision? I know that she cannot remarry for at least 90 days (this source says 92). Must he decide by then, or can he delay beyond that point?
I am asking about a brother who is of age to marry.